IMac crashing and display problems

Question

I've been getting weird graphical glitches while playing World of Warcraft, such as this kind of thing: http://img208.imageshack.us/img208/1582/piccie10gl.jpg
Also, occasionally my computer will completely freeze up, or at least the monitor does. Everything will just freeze except the cursor, but I continue to hear sound as if it hasn't frozen. Once this even happened while I was burning a CD, and as I could hear the CD continue to burn I left it for a few minutes and came back and forced a shutdown and then restarted the computer, and the CD had finished burning.
Other times the screen will go black, and once it even flashed a dim gray color on and off.
I haven't ever seen this kind of thing before so I'm at a loss as to what to do about it. It seems to happen most when I'm playing World of Warcraft. Could it be the graphics card? I have an upgraded card, 256mb I believe.
I also have a very full hard drive at the moment (about 85-90% full, 250 gigs) which is the only thing that has changed between now and when this problem started.
Anyone experienced this problem before and/or know what is causing this?

Answer 1

Hello everyone.
This was already posted on http://discussions.apple.com/thread.jspa?messageID=7359296#7359296
I had the exact same problem, and it was fixed after Apple replaced the logic board (motherboard) on my white 24" iMac 2,16.
It started last year with the exact same issues already discussed here as well as in http://discussions.apple.com/thread.jspa?threadID=1441391&tstart=0 . Back when I bought it, I went to the Apple retailer store to ask about heating problems, as I thought my machine wasn't operating the fans correctly. They told me they knew of no problems of this kind, so I didn't do much about it after that.
About 6 months of World of Warcraft, iMovie, Photoshop, Youtube, Garageband, web browsing, word processing, iTunes playing, sometimes some of these all at the same time, my computer started to freeze once a day, then twice, 3, 4 times, and graphic tearing started showing up, first in WOW, then all over the screen. After some extensive reading here on these very same forums, I realized my logic board, being constantly subject to +80º C temperatures, simply started to fry up. One day, my iMac simply did not boot up at all.
Now its working flawlessly but again, temperatures get to over 70º, sometimes 80º, and the fans don't go anywhere higher than 600-1000 RPM; I monitor temperatures with Temperature Monitor (free app) and use smcFanControl (also a free app) and manually turn up the ventilation when I'm using Warcraft (5 hours a day). No issues so far.
The temperature on both processors, the GPU diode and specially in the power supply position is messed up, I believe by faulty controllers that don't manage the fans properly, leading to hardware malfunctions. Sadly, I have to say Apple, after turning to intel, has lost most of its reliability in hardware. My former iMac G4 never had any problems for 2 years (used Warcraft III there as well, and iMovie)
Anyone with these issues, if your computer started freezing up and showing graphical corruption, its most likely a logic board slowly dying. Take it in for repairs as soon as possible.