Bad video card or display?

Not sure about the dark screen effect, but a general slowing could due to
an increasingly fuller hard disk drive; though that may not be the case.
Since less free-space on a computer's hard disk drive could mean it would
have to work harder applications would be fighting each other for work space
on the hard drive, about the same time as the OS would be wanting more.

And free space on a hard disk drive is also used as Virtual Memory in OS X.
So, if your Activity Monitor and other built-in system gauges tell you of these
kinds of things, and page-outs, excessive CPU usage, less than 20% free-
space on the hard disk drive (hard to imagine, but it happens) then the issue
could be, time has come to clean out the iMac and save your stuff off the
computer's hard disk drive. Images and music, and then figure out how to
delete the originals from their places in the applications & other hiding spots.

Some applications such as iPhoto have their own trash, and some apps
make duplicates, so you could have quite a bit on a hard disk drive.

There is an Apple Hardware Test on the bootable DVD installer disc, and
you can access that by inserting that disc into the computer's optical drive
and then restart, while holding down the Opt key. This should get you a
showing of what is on the installer's DVD. The Apple Hardware Test is one.
{This would be available on the original software disc included with the Mac.}

Also, if you use that installer DVD (or a more recent one, if the system has
been upgraded since the computer was bought, to a new OS version) the
Disk Utility on the booted installer can try and repair the disk; and also tell
you if the hard disk drive is going bad, since it can evaluate SMART status.

If your computer still has an active AppleCare protection plan in effect,
then after trying and finding out the status of the computer, you probably
should make sure all your files and stuff is backed up, and then call Apple
Care and see what they can help you with. The extended AppleCare plan
is a good investment, and it adds two years to the original first year; if
bought and enrolled before that first year of ownership is gone.

Good luck & happy computing! 🙂

Your iMac is probably overheating. Download smcFanControl.

It helps, but you should also complain to Apple because they do not receive any crash reports from your dying Mac.

You might have to keep tweaking those minimum values depending on the tasks you're performing, as the machine doesn't seem to adjust the speeds by itself.

You can tell what is using the processor, hard disk drive free space,
RAM, and other heat-causing and energy sucking bits in the OS by
using the Activity Monitor; and alternatively if these are using way
too much CPU cycles and causing heat unnecessarily, this may tell
you a few things about the computer.

And if you do not perform systematic preventative maintenance on
the OS, some of the same issues can result. These basic things
are about the same whether or not you happen to have an Intel®
Mac or a PowerPC series Mac; OSX after 10.2.8 has its needs.

The computer could be overheating; and an idea would also be to
get some software that'd put a temperature gauge in the menubar.

There also are some nice widgets, but they too eat up resources
and can make the processors cycle faster or hotter. I've found one
small widget that can kill all the dashboard widgets at once. Handy.

Good luck & happy computing! 🙂

Yes, I use [iStat|http://www.islayer.com> and [Temperature Monitor|http://www.bresink.com/osx/TemperatureMonitor.html].

I'm not sure what preventative maintenance you're referring to. The iMac should be able to run 3D OpenGL and other high-performance computing apps without the user having to worry about heat issues.

All I know is that my machine had choppy graphics during 10.5 and 10.5.1, and then it went downhill since 10.5.2 with horizontal lines and endless beachballs, albeit with smoother graphics. On the other hand Tiger was flawless during its 10-month run inside my Mac.

It wasn't my computing habits that changed. It was OS X.

Given my older G4 Macs shipped with earlier OSX versions, and I upgraded
the iMac G4 1.25GHz 17" last model to Tiger after deciding that I'd like it
to get newer security updates and they'd stopped supporting Panther... and
my iBook G4 1.33GHz 12" last model came with Tiger. Both of these are
stable and well enough at 10.4.11 and given all the updates available.

{My G4s have those temperature utilities; among others. An advantage of
having at least one machine a bit ahead of the other one has been to
find and use the better technology, then adapt to it; when it was refined.}

And these will not see a later OS X unless evidence suggests a better
experience with them may be worth the extra effort, expense and trouble.

{Using tools such as AppleJack or more simply, OnyX, to clear caches,
perform cyclic routines, repair disk permissions, and other stuff which
could be done in a single user or terminal, etc, mode; helps a Mac run.
Also, after some hundreds of hours of use, to have a complete clone
of the computer's drive, then wipe the internal one, zero it, and then test
and repair the disk drive; and clone back the system & content, helps.}

My next computer will probably have Intel® processors; although I preferred
PowerPC architecture and the earlier lower power RISC system logic... And
the newer hardware will be better suited (by then?) to more than handle what
ever I may be willing or able to throw at it. And I am not in a rush atm.

Some video cards had issues in some configurations; but to find evidence
of this may be hard. Not all users experience the same kinds of issues at
the same duration of use; just like 'some model users' had problems in only
a few of the random models of G5 PowerMacs, or other series. Not all.

Some users found to wipe the hard drive, use Disk Utility to overwrite the
content with zeros, and reformat, then put an all new installation or a clean
complete clone back into a computer, can make it run cooler and better.
Plus remove any huge image or audio files to a clone external HDD or to
DVD media instead of filling the internal hard disk drive. Drives get hot, too.

Good luck & happy computing! 🙂

{PS: did you know this is in the iMac G4 section of Discussions? I did.}

edited to add more words.

I don't think I'll be reformatting my hard drive anymore. All that activity might make it worse. What I do want is a rollback of the firmware to its original state.

My PowerBook G4 has survived Panther, Tiger and Leopard without resorting to AppleCare. How was I supposed to know I would need AppleCare for my (Intel) iMac?

(Yes, I'm aware that this is the wrong forum. I found this thread by using the search.)

Just letting you know that I had already tried wiping my hard drive and restoring Tiger, but I still see the same problems. The only thing that I have yet to try is removing all of the firmware updates, but Apple doesn't even allow us to do that.

This is starting to look like an irreversible damage.

That's OK. I don't mind; but some other forum replies I've read
indicated they'd not even post the courtesy of a reply outside
of the 'correct' forum for the hardware questions asked there.

Some of the same issues appear under the OSX version forum
and also in others, just so one user does not post duplicates;
so if you do need to rephrase your question, it would work in the
other location and not conflict with this one.

Am I correct in the assumption your Intel® based iMac does not
have an active AppleCare protection plan in effect anymore?

Apple Support> Discussions> Intel® iMac forum section:
http://discussions.apple.com/category.jspa?categoryID=189

Firmware restore Intel iMac 24" - google search:
http://www.google.com/search?hl=en&q=restoringfirmware+Intel+iMac+24%22&btnG=GoogleSearch

This issue certainly sounds like a hardware failure to me, sadly.
Best of luck in this matter, in any case!

edited to add link.

Thanks, but you can only apply the restoration after a previous update failed. And I think those CDs contain the latest firmware. If only I knew how to zap it somehow...

You're probably right. It's just doomed. I think I'll take it apart when I have the time. So depressing..

You have to pick a specific forum like [Your Intel-based iMac Display|http://discussions.apple.com/forum.jspa?forumID=1114]. This isn't a new problem. Take a look at this [thread|http://discussions.apple.com/thread.jspa?threadID=1467276].

Yes, I would take it to a [Genius Bar|http://www.apple.com/retail/geniusbar> and demonstrate the problem. I don't know if they'll let you upgrade. Let me know what they replace.

Once you install firmware, it stays inside those chips until a later version replaces it. I think the installer enforces the version number.