My 20' G5 iMac ran beautifully with Tiger. Hubris made me install Leopard. The install went flawlessly. Since then, every time that I wake the machine from sleep the system crashes. I have tried to put the machine to sleep manually without any change in the crash. I have tried different ways to wake the machine, ie pressing different buttons, moving the mouse, etc, and nothing works. I have seen from the Apple discussion forums that this is not an uncommon problem. Further, there are rumors that the ATI video card may be the culprit.

Can anyone please help? Please?