2001 G4's and their installer CD's automatically place drivers for both NVIDIA and ATI AGP video cards. The OS (and OS BASE set) have both sets of extensions on. Determine which AGP is in your G4 (Use Apple System Profiler). Then use Extensions manager to disable the drivers for the card you are not using.

If this does not help:

Rebuild Desktop : hold Option and Command(apple) while booting the G4
Zap PRAM : hold Option, Command, "P" and "R" while booting.
Reinstall Mac OS "in place", saving original items.