It's looking like it varies from computer to computer and how it is configured. My friend and I both run G4 iMacs with large screens. We just happen to experrience the same exact problems, so we can assume from our micro-consensus that G4 iMacs may have some intrinsic problems. That's the most I can share as to specifics beyond what I've already stated.

I've run DiskWarrior, TechTools (the complete over night one) to no avail. I'm afraid I'm going to have to do a complete wipe and reload of my system, apps and all that other stuff, from scratch. (Not a happy camper, as the saying goes.)

Hope this helps.