I have experienced the same problem but have also had a problem that seems to occur when the USB port is being accessed. I have a HotSync cradle for a Visor and an HP printer that both use the USB port and both have caused reboots.
I am running;
Abit KA7-100, Athlon 700, 128MB Micron Ram, 2 Hard drives, 1 CD-Rom, 1 CD-Burner
Full Tower case with 300W Smartpower, 3 Case fans
SBLive Value, VooDoo 3000, Cable Modem (PCI nic)
Any advice would be appreciated.