Help, imac 2008 3.06 display problems

[gez]

I started having a pixelated and scrambled screen when using aperture or iPhoto which caused the machine to crash. It then started to happen when it was left on for a few hours.
I assumed it was the graphics card (8800) due to the history of this card. I got this replaced and it's still happening. Interestingly the repair place cannot get the problem to occur with all the hardware tests or running off an external disc.
Anyone have any clue what's wrong?? Photo attached.
thanks

 

[volcomfever]

I started having a pixelated and scrambled screen when using aperture or iPhoto which caused the machine to crash. It then started to happen when it was left on for a few hours.
I assumed it was the graphics card (8800) due to the history of this card. I got this replaced and it's still happening. Interestingly the repair place cannot get the problem to occur with all the hardware tests or running off an external disc.
Anyone have any clue what's wrong?? Photo attached.
thanks

The next thing to do is to verify this happens in a known good OS. If it does, you can eliminate software and know for sure it is a hardware problem. If it is a hardware problem and since you already had your video card replaced, next thing would be logic board most likely.

If it does not happen while booted to a known good OS on an external device, then it is software and you should do a clean install ASAP to get rid of the problem. And just to verify, this scrambling only happens once booted into the OS and not during the grey boot screen with the apple logo?

 

[gez]

Thanks for the reply. Yes it has only happened in booted os and not in boot screen so far. The system has run well largely unchanged for a number of weeks which is why I thought it was a hardware issue. Aperture and iPhoto both create error after moving through a few photos which is why I thought it was the graphics card...it seemed to start off when left ripping a DVD overnight.

 

[volcomfever]

Thanks for the reply. Yes it has only happened in booted os and not in boot screen so far. The system has run well largely unchanged for a number of weeks which is why I thought it was a hardware issue. Aperture and iPhoto both create error after moving through a few photos which is why I thought it was the graphics card...it seemed to start off when left ripping a DVD overnight.

Yea the next step is definitely to boot to a known good OS and do the same things that caused it before and see if the issue pops up. If it doesn't, it's definitely software related.