I just facing this today also! My Core 2 Duo iMac suddenly turn off when i listen iTunes just 15Min..... when i turn it on again , the status light keep blinking! After 15Min, i try to turn it on, it's fine and i switch to XP , play NFS Carbon about 20Min, It go off again, and the status light keep blinking!
I was get shock , what problems izzit? just 2 months using? I suspect RAM problem, So i open the ram slot, when i take off the ram cover , 2 ram drop out from there ! OMg.... the ram already lose and trying to came out , lol..... I fix it back and close it back, it's fine and stable now .
izzit your iMac maybe ram problems?
already sent it to the center so wat i can do now is wait them to make my imac work then pay loh..