iMac freezing

Im having problems with my iMac freezing a lot.. If I leave it turned on and return after a few hours I often find it is frozen. When I touch the mouse I get grey pixels where ever the curser goes which slowly reveals the login page. I type my password and it reloads everything. I can see no obvious connection with a specific program, it seems to happen when I leave various ones running/open. Can anyone offer any suggestions or advice?
Thanks.

Which version of OS X are you running on the iMac? Reason I ask is it might be a good idea to do a check of the hard drive. If you're running Snow Leopard, boot with your SL DVD and run Disk Utility to verify - repair the hard drive. If you're running Lion, boot to the Recovery partition and run Disk Utility from there.

You will have to excuse my ignorance. The iMac is about 6 months old and I don't recall it coming with an installation disc. The Software is Mac OS X Lion 10.7.3 (11D50b).

it most probably comes then from the app store. i´d say restore the files for lion. or start deleting your caches and repair permissions.
here a detailed instruction:
TheMacMania - Tips, Tricks and more for your Mac Computer

actually i'd try that first before replacing lion

@britclin

Look at the last sentence of my reply:

If you're running Lion, boot to the Recovery partition and run Disk Utility from there.

Click to expand...

Lion does not come on disk - and that's why I mentioned the above in my first reply. And why it's important when asking for help in these or any forums to provide us with information about your Mac and which version of OS X it's running.

Once you've done the maintenance described by TheMacMania and chscag, if you continue to have problems I would strongly suggest taking it in to an authorized Mac dealer for a checkup -- could be a hardware issue and you still have AppleCare so let them handle it.

Thanks for your help to date. I ran the disc utilities and the disc verification says everything is ok so I started doing what Macmania suggested and got to the stage of emptying the wast bin and got the following message. "There are some locked items in the Trash. Do you want to remove all the items, including the locked ones, or just the unlocked items?" This is a little different from the message that you suggested I would get, how do I proceed?

that's an easy fix - go to finder --> secure empty trash - that should solve the problem.

I finished deleting the caches and emptying the trash but unfortunately I'm still having the freezing problem.
I apreciate what chas-m said about taking it to a Mac dealer but I live on Lanzarote and there isn't one on the island so if anyone can suggest another possible solution i would be happy to try it.
Thanks for your help everyone.

i´m sorry to hear you are still having problems. well, another idea would be to run the hardware test from apple - to make sure that your hardware is just fine.
my next step would be to re-install the OS
but before you do that - maybe a ram/hardware reset helps? i'd try to reset all i can before i would re-install the OS.
what are you doing exactly before it freezes? maybe it´s "just" a program causing this?

Hi again. I think I have sorted the problem, I removed a piece of software called TuneUp; it was installed about 4 months ago and used for a week or so and then closed and not opened again. It was the onyl piece of software, apart from Microsoft Works, that I had installed so I thought it would be worth uninstalling it. I have waited three days now and not had any more problems so I'm hoping that is the end of it. I wouldn't mind using this program again in the future, can anyone suggest a remedy.
Thanks again to everyone for all your help, it is much appreciated..

hi

thanks so much for getting back about this. it´s really strange as i also use this problem and don't have this issue - but 100 configurations = 100 different problems oh well
glad you sorted it out, next time my computer gives me a hard time i will try to uninstall this software first