I agree with patio that it's likely a software issue, of which drivers is one possibility. But since it's not a driver let's see if it's some other process. Open msconfig and choose "selective startup" (disable everything) and reboot. If the issue recurs then it's not software. But if it does not recur, start adding items back one at a time, rebooting after each, until the problem does reappear and you'll have identified the culprit.