iMac GPU going out?

Looks like it.

do syslogs report anything?

Looks like my iMac when the GPU was toast.
Did a reflow and everything was back to normal.

http://www.overclockers.com/forums/showthread.php?t=606658

Search google+reflow gpu

Could be an overheating gpu. Maybe by dust. Use a fan control panel to rev up the fans and try to blow out the dust. This could do the trick, but don't get your hopes up.

mogens said:

Looks like my iMac when the GPU was toast.
Did a reflow and everything was back to normal.

http://www.overclockers.com/forums/showthread.php?t=606658

Search google+reflow gpu

Click to expand...

A temporary fix at best. I guess everything is "temporary" to a certain extent. However, I would never recommend a reflow for long term reliability. A faulty chipset or defective part is going to continue to generate an insane amount of heat. Before you know it, your GPU will be doing the same thing again and in need of another reflow.

I'd stay away completely. Replace the part with a new component with an updated revision. If no updated revision exists, it's time for a new machine.

Just my two cents.