Black screen and loud fan noise

I am running 10.5.8 on a second gen iMac and am having problems with the fans running super loud, the screen goes black and it cannot be woken up. I have cleaned it out (no dust or dirt) and checked the caps (visually anyway) and they look fine. I am guessing that my power supply is going, but want to check the log (as recommended on the iMac forums) but cannot figure out how to get there. Any advice is welcome. Thanks in advance!