I have an iMac G5 2.0 GHz, ALS, running OS 10.5.8, about 3.5 years ago I upgraded to 2 GB RAM, and 2 years ago a 1 TB HD. The original 250 GB HD (that was running OS 10.4.11) is in an external case and is attached to the iMac via firewire.
About four days ago my wife was using the iMac, stepped away for a half-hour or so and came back to it. When she awoke the iMac from the screen saver, the screen had turned kind of a pink-pixelated-geometric pattern that you could kind of see through. The pixelation kind of runs horizontally and vertically in a pattern. You can go to the dock and start Mail or Safari and they work, but it is hard to use because its hard to see through the pixelation.
For reasons to long to get into here, when I installed the new HD, I partitioned it, and installed OS 10.4.11 on the second partition.
I tried restarting the computer. That didn't work. I tried resetting the PRAM, that didn't work. I tried restarting from the new 10.4.11 partition that didn't work. I restarted from the original HD, and that seemed to work. The screen returned to normal. I figured my new HD was going bad. Just to make certain, I restarted from the new HD (running OS 10.5.8) and screen was fine there too. Now I was confused.
So I got on to this discussion forum, read through several posts that seem to relate to screen problems, and found a few possible solutions if the situation happened again. Well tonight, it happened again.
I shut the computer down, disconnected all the cables and opened up the back cover. All of the capacitors looked fine. While I had it open, I sprayed the dust out of it (it wasn't very dusty as I do this about once a year anyway), put the back on, reconnected the cables, and reset the PRAM.
I restarted the computer and the screen was still pixelated. I restarted in safe mode, still pixelated. I restarted off the old HD, still pixelated. This is a change from before, now thinking its not the new HD. I restarted from my OS 10.5 install disc, still pixelated. I tried running disc utilities, first aid, but I couldn't really read it, so I stopped before I did something stupid.
I restarted again from the old HD (running 10.4.11) and I tried a possible solution I found suggested on this forum. I opened the icon the HD with the 10.5.8 system, and then I opened the folder /System/Library/Extensions, and I removed all the ATI extensions, and placed them in a folder I created called ExtensionsDisabled. I then placed that new folder back in the /System/Library folder, and restarted the iMac using the new HD and the 10.5.8 partition. Everything is still pixelated.
I know I have the original disks that came with the iMac somewhere, but at the moment I can't find them. According to the Spec page on the Apple site for this model iMac, the Apple Hardware Test disc was supplied, probably from what I understand, the Apple Hardware Test is on the second install DVD that came with the iMac.
I will continue to look for those disks, but in the mean time, does anybody have any other suggestions for solutions or possible causes?
Thank you very much in advance.