System rebooting - on its own

Lately, while online, my screen would suddenly go black and the system would
reboot. A virus scan showed no viruses. There seems to be no pattern of
this activity. I could be online for a couple of hours or minutes. There
were no ''flags'' warning of coming trouble. And I've experienced sluggish,
jerky mouse pointer movement. I have also experienced website sound files
playing ''broken up'' a bit at times. I have plenty of ram. Using IE6 and
DSL; the DSL speed is still good.
Ideas on this?
Thanks.
Bob