I have had a similar problem on my G4 two or three times (dual 450MHz, 768MB RAM, two partitions - one for X, one for 9.1). The problem does seem to be related to Classic, in that the machine seems fine until I have to start using Word, Fireworks, etc.
I suspect that the problem is hardware related, since my Pismo powerbook is rock solid (384MB RAM, again 2 partitions). Classic works, everything's stable - I've had it running continuously for over a week of moderate to heavy use.
Has anyone else had a similar experience?