Grey screen. Won't boot, it says it needs a restart.

Greetings.

I have a iMac A1200 (2.16Ghz, 1Gb RAM). It turns on just fine, but after the Apple logo it shows a warning telling that I must restart the computer (grey screen).

After reading some advices posted, couldn't find the appropriated answer.

I tried to boot in safe mode, it stops at "DSMOS has arrived". Also it says "Previous Shutdown Cause: 3".

I can't insert any disk.

Finally I disassembled it to see if maybe it was a problem of some blown capacitors. All of them look perfect.

So, do you have any idea what can it be? Is there a way to know for sure if it's a logic board issue or graphic issue?

Thanks for your time.