I had a similar problem: every time my Panther would sleep and I waked it up (after 3-4h) it froze completely. Sometimes it happened directly at start up and a it asked me to restart in many languages. I figured out that it was a "kernel panic", which was due to a faulty 512mb memory card (Kingston). I pulled it out, tested it for some tim. Everything was perfect allthough a bit slower naturally. Then I changed my memory card for a new one, pulled it in and everything worked beutifully.
Now my Panther is both very stable and pretty fast, I love my Mac!! My Mac hasn't crash even once since i.e. in 1.5 months :-)