Computer freezes when rendering

G4 Quicksilver
1.7 MHz Processor
1.5 GB SDRAMM
FCP 6
OS X 10.4.11
Quicktime 7.4 Pro

This has been happening only since I upgraded to FCP 6. Even on the smallest clips, when there is heavy filtering, the computer freezes up during rendering. I am stumped. This never happened in FCP 4.5 HD or FCP 5. Any ideas? I have plenty of SDRAMM and no other programs are open when this occurs.