iMac freezes after ~10 minutes. Repeating blue dots appear on display.

[rentacop]

I have had my iMac for about 2.5 years. I am currently running Snow Leopard.

When I go to use it, I can only use it for about 10 minutes or so until the screen freezes up, and small, light blue "dots" appear all over the display.

If I turn it off and then turn it back on again, this screen appears (see attachments). If I wait around two minutes instead of turning it off again, the dreaded gray kernel panic message appears.

It should be noted that none of these problems occur when I start the Mac in Safe Mode. I originally though it was an overheating issue, but I tried it a couple times today, and the Mac doesn't seem warm at all.
Does anyone have any ideas of what could be going on? Thanks.

 

[Intell]

That would be a defective video card. Overheating is very, very rare in an iMac and likely to not be the cause of that problem.

 

[RustyMacVet]

This is what the future of computing looks like. The dots are command modules.

 

[dany74m]

That would be a defective video card. Overheating is very, very rare in an iMac and likely to not be the cause of that problem.

I agree, I had a similar problem with my old 21" White Imac.

 

[aarond12]

That would be a defective video card. Overheating is very, very rare in an iMac and likely to not be the cause of that problem.

I strongly disagree.

My wife had a similar problem with her 24" iMac (white casing). She was using it and I just happened to look over and see some artifacts. I had her shut down the computer immediately and the computer was quite warm to the touch.

I took that bad boy apart and, sure enough, the computer was full of dust. I did a thorough cleaning of the computer and it's working fine again.

So get out the suction cups...

 

[Intell]

iMacs have a very well designed cooling system. This is evident by them rarely revving up their fans. Aluminum iMacs are always warm or "hot" to the touch because their bodies act as a big passive heatsink. This is by design to help with the cooling.

 

[mdgolom]

This is very interesting. I pretty much had the same problem with dots and blocks showing up on the screen and the system crashing. I reformatted everything and it still had the issue. I'll have to look into opening it and seeing how dust it is. I'm going to guess that it is very dusty after 3 1/2 years.

After I took the computer some from where where I reformatted it, I have only had it crash once. I ran the hardware diagnostics and it passed. When I had it run the full in-depth diagnostics it froze after 11:32 on the memory test. I decided to spend $50 and upgraded the memory to 8GB. So far it hasn't crashed/froze again. Trying to hold out for the new iMacs.