Crash woes- but what causes this?

I have recently tried Knoppix for the first time, and I must say, it is impressive. I'm liking it much more than windows. However, there is a problem. The first time I loaded it, it worked pretty well for about an hour, and then it crashed to reboot. Now, it crashes in under 8 minutes from loadup. I ran memtest86 and my memory is okay. Knoppix doesn't report any error messages in loadup.

Currently:
Zoomlink modem
256mb ram
sis300 video card
1 cd-rw (Que!, I think.)
2 HDD- one maxtor(40), one seagate (80)

Even knoppix failsafe will crash my computer no matter what I do. What's the source of my woes?
Thanks

Try booting like so:

knoppix testcd

It should return some thing like: "everything looks OK"

If testcd is ok then tell us what happens exactly when it crashes. Does it freeze or simply reboot on its own?

Testcd reports everything looks OK, but when I shut down, I did notice this error (Mostly correct, some parts a bit paraphrased):
sisfb: Fatal error: unable to reserve framebuffer memory.
sisfb: Is there another framebuffer driver active?
sisfb.o: init_module: No such device
This can be caused by incorrect module parameters including invalid IO or IRQ parameters.
insmod sis failed.

This popped up just before the graphics part loaded.