I am having a strange time sync problem & running out of ideas.

I have 4 NW 6.5 servers in my environment, one of which (Server A) is
configured as a primary time server that syncs against two NTP servers
outside my organization. The other 3 (Servers B, C, D) are all
configured as secondaries and all sync against Server A.

Servers B and C are fine; no issues whatsoever. They poll the primary
timeserver every 600 seconds as expected and typically make a small
correction of 10-20ms either way.

Server D is having problems. It fails to sync approximately every 3
polls, and when it re-establishes synchronization on the next poll, it
is making a correction of anything from 1100 to 6500ms +/-. It then
continues to poll every few seconds, always making a correction of at
least 1100ms either way, until it loses sync again 3 or so polls

I suspected a cmos battery failure but the hardware (Dell) is not
indicating any problems, and the server itself is less than 2 years
old. (In fact all 4 of them are.)

Any thoughts?