iMac screen goes black - have to power down

Sorry, I know this comes up in all the forums but I'm not finding answers. While I'm using my Mac the screen goes black. I have to power down [control-opt-command + power button], wait, & power on again. It's becoming more frequent. I'm becoming more anxious. Thanks.

iMac early 2008
2.66 GHz Intel Core 2 Duo
6 GB 800 Mhz DDR2 SDRAM
ATI Radion HD 2600 Pro 256 MB
OS X 10.8.2

Sounds like the backlight is failing. In this thread from Apple Discussions, go to the ifixit link:-

https://discussions.apple.com/thread/3760852?start=0&tstart=0

More info on the problem

Thanks, but maybe I didn't include enough info. In its early stages, the screen would suddenly go black...or vertical red stripes or yellow or occasionally gray. I upgraded to 6GB memory (as recommended somewhere online) & I've had no more colored versions. The sudden black continues. A repair shop told me it's the logic board, and I truly can't afford to replace that. Does this history make a difference? I ask because the thread you referred me to addresses dimming rather than black-outs.

You may be correct in logic board. Suggest not worth spending big bucks on now five years old.

I agree in that it sounds like the graphics processor on the logic board is failing or intermittent. And since the GPU is soldered to the logic board it means replacement of the entire board. Not worth repairing on a machine that old.