Your Windows 2000 or Windows XP Performance Monitor records CPU usage that is inconsistent with the computer activities during the sampling period? The CPU cycles may vary from 2 to 30 minutes of high or low CPU usage?
During the periodic sampling, if the activity synchronizes with the sampling interrupt, this behavior will occur.
If the a client sends consistently timed packets and the clients clock drifts slightly from the 'server' clock, Performance Monitor may report alternating high and low CPU usage on 'server'.
Microsoft is aware of the problem.