I have a iMac 20" running Leopard with the latest updates. When not in use but powered on, no problems. When in use (either Safari or ARD), lasts about 30 minutes and it loses power and does not restart even though I have it set to reboot on power failure. My thoughts are RAM or power supply. I ran the hardware test from the disc, no problems and it lasted 45 minutes. I ran the TechTools disc and again no problems. That makes me think Power Supply. The symptoms above suggest RAM. I have my Apple Desktop and Laptop certs so I can order parts, but I'm just looking for ideas or if anyone else has ran into this. Thanks guys.