It wouldn't hurt to unplug all the USB goodies and see if that helps. I have had similar problems with my (much older) mac for a couple of reasons:

1. Old PCI cards that aren't compliant with the power managment. Sounds like this probably isn't your issue.
2. Overheating. My beige G3 will overheat and crash while asleep if the ambient temp is above 85-90 degrees. My box is very loaded (3 PCI cards, 3 hard drives, etc) so this isn't that surprising. You might want to try blowing out the inside of your case and power supply with some canned air and see if that makes any difference.

Just a couple guesses.
-Mike