NT server cpu useage is 90+%

Until last week, our PDC, running NT4 SP6a ran just fine. Then it basically locked up and after reviewing the Task Manager saw that the CPU useage was 90+ percent. The system process was the culprit as it was grabbing 90+ percent of the CPU. Subsequently, after a reboot the server ran fine for about 4 days and the same thing happened. Rebooting cleared the problem up. I haven’t installed any new hardware or software on this server. Has anyone experienced a similar problem and if so, what was your resolution? Without having done anything the problem is sure to rear itself again.

TIA