Causes for iMac G4 to freeze up.

My iMac G4 1.25 ghz, 2 gb of ram, 17 in model number M9168lla failed to load OS 10.5, froze 20 min into install, I replaced hard drive with a 320gb, in fire wire target mode I installed 10.4, once finished I restarted the iMac and it ran nicely for 20 min, then froze up. Any ideas as to what causes this?

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