A PC generates a 'timer interrupt' about 20 times a second, and the
clock is maintained by counting these. Clock rates that run steadily at
a big loss or gain are probably because the motherboard/BIOS and Windows
have different ideas on how often the interrupt happen. Small
variations in this are handled when you use the Internet Time sync -
this notes the error and adjusts the assumed interval accordingly, so
that after a few occasions the clock rate is very near right anyway.
The interrupt mentioned here would that be caused by software or hardware hicup?

That sounds like what his problem is...