Are there any known problems with clock drift?  How about under heavy 
network load?

Do clock interrupts get lost if there is "too much" network traffic?
Or does xntpd get confused?

mckinley% grep "time reset" /var/log/messages
May 27 16:32:54 mckinley xntpd[167]: time reset (step) 1.477446 s
May 27 16:38:36 mckinley xntpd[167]: time reset (step) 1.751542 s
May 27 17:13:27 mckinley xntpd[167]: time reset (step) 0.419863 s
May 27 20:34:10 mckinley xntpd[182]: time reset (step) 117.880780 s
May 27 22:54:02 mckinley xntpd[182]: time reset (step) 107.326875 s
mckinley% 

I'm not too surprised by clock troubles but I don't see any reasonably 
way for the clock to get off by 100 seconds.