G4 Powermac Constantly Freezes/Crashes after upgrade from 9.x to 10.3.5

It'a an AGP Graphics Powermac running at 400 MHZ. 256 MB PC 100 RAM. The system freezes constantly, requiring re-start. I've replaced the RAM, and meticulously re-installed the O/S numerous times. No specific or processor intensive task or combination of tasks causes it to freeze, it's completely random -- be it opening a document, moving a window, etc.

I've run disk verify/repair a lot, and I've run Norton Disk Doctor and SpeedDisk as well. Still the darn thing crashes like crazy. This never happened under O/S 9, which makes me want to doubt it's a motherboard related issue.

Ideas? Comments? Anecdotes? Solutions? Anybody?
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brettmjohnsonConnect With a Mentor Commented:
It sure sounds like a HW problem - typically questionable RAM or CPU overheating.
Mac OS X is much less tolerant of poor quality RAM than OS 9 was.

weedConnect With a Mentor Commented:
An older version of Norton Disk Doctor will wreak havoc on OS X systems. THAT could be a problem. RAM could be a problem since youve already given it a fresh install, but I'd try WIPING the drive, installing OS X, and NOT run Disk Doctor or Speed Disk.
idarmadiConnect With a Mentor Commented:

Don't forget to check if there's any firmware update for your machine.
mbarkdullConnect With a Mentor Commented:
Those Macs came with the Apple Hardware Test.  If you don't have that CD anymore, I recommend TechTool for testing it.

You can boot from the CD and run it in a loop to test hardware.

Make sure the fan is turning, heat can be a problem too.
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