Take the back off by unscrewing the three screws on the bottom of the frame. Inspect the capacitors (cans) for corrosion and/or bulging. Also examine the GPU for stress fractures. If you find it and don't feel comfortable repairing it yourself, I found a guy on eBay that does it for $75. http://cgi.ebay.com/iMac-G5-Logic-Board-Capacitor-ATI-GPU-Repair-Service-/250632979482?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item3a5ae3c41a