I'd be really grateful if someone had guidance to offer!
Just in case the added details are useful:
Model Name: iMac
Model Identifier: iMac8,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.8 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 6 MB
Memory: 2 GB
Bus Speed: 1.07 GHz
Boot ROM Version: IM81.00C1.B00
SMC Version (system): 1.30f1
I have the same model. What I suggest you try next for this problem:
1. Run the Apple Hardware Test. It can't detect all hardware problems, especially intermittent ones like you might be having, but it is still a good first step.
2. Reset the SMC. I'm not sure why but this has fixed some odd problems I have had in the past with my 2008 iMac.
3. If this doesn't help, try running for a while without any peripherals attached to your iMac besides the keyboard & mouse. If you don't see any freezing problems, try reattaching your other peripherals one by one & running that configuration long enough to see if one of them is causing your problems.
4. If none of this helps, go ahead & reinstall Snow Leopard, making sure you run Software Update afterwards to update it to the latest version. The Snow Leopard installer isn't like earlier OS installers. To reinstall the system just run it & accept the defaults. It won't write over or alter your user data or create any "Previous System" folder stuff, so it is a fairly painless procedure. It even performs the equivalent of Disk Utility's verify disk step, so you don't have to do that beforehand.
5. If none of this helps, reply to this topic & we can suggest other things to try.
Hmm, so tried steps 1-3 above and my Mac crashed again today without warning. I was using only firefox at the time and had no peripherals attached. Before I go on to step 4 and reinstall Snow Leopard, can anyone advise whether I would be better to try upgrading to Mountain Lion instead, and see if that helps? I figure I'd need to upgrade at some point anyway so might as well do it now rather than reinstall Snow Leopard?
Advice gratefully accepted!
Some more suggestions:
1. If you haven't already, back up your HD as soon as possible.
2. Try creating a new user account and using it to surf the web, etc. Each user account relies on thousands of small files to run properly and it's not uncommon for one to become corrupted.
3. Boot from the System DVD that came with your computer (instructions are on the disc label) and then use Disk Utility to check and repair your HD.
Thanks all for the suggestions!
@ inandoutofgrace - I have backed up the HD, will create a new user account and work on that tomorrow and see if there's any difference. On the Disk Utility suggestion, I've done this already, as did the Genius Bar, accessing it through Disk Utility directly from Finder. Will running it from the System DVD be any different?
@ seventyone - I don't think there's much running in the background. I'm usually not doing anything other than running Firefox and Word. When I check on Activity Monitor, it shows low usage (often 98% idle under CPU) and that 1.18GB of my 2 GB system memory is free.
@gswg1968 - Have just reset the PRAM so will see if that helps.
If there's any further info that would be useful, feel free to let me know. I'll see tomorrow how the different user account and PRAM reset work out. Thanks for all the help
"Will running it from the System DVD be any different?"
Ordinarily, running Disk Utility from the Finder (i.e. using the copy of Disk Utility that is installed on the HD the computer is booted from) allows verifying, but not repairing, the HD.
Running Disk Utility from the System DVD will allow Disk Uitility to do repairs on the HD.