I have a Leopard family license DVD, which I have installed successfully on four of my other Macs (including an iMac iSight). My other iMac ---a pre-iSight G5--- will accept a Leopard install and perform flawlessly ---even for weeks--- until I shut it off or attempt a restart. Then I get the startup chime, followed by the gray screen with the spinning cog, the fans eventually go into hurricane mode, and the computer powers down before automatically repeating the sequence endlessly. I can start from the Leopard install disc and restore using Time Machine, using the computer as normal until I have to interrupt power for any reason (like a software update), at which point I have to endure the 2-hour restore again. I have tested, erased, and zeroed the internal drive, zapped the PRAM, and reset the SMU... I have NOT run a hardware test but since it worked perfectly with Tiger (and is otherwise healthy under Leopard) I assume that the hardware is intact. Any suggestions for a remedy? THANKS!