[CentOS] clock sync/drift

Hi,

We have a little over 100 servers, almost all running CentOS 5.7.
Virtually all are Dell servers, generally a mix of 1950s, R610s, and
R410s.

We use NTP and/or PTP to sync their clocks.  One phenomenon we've
noticed is that (1) on reboot, the clocks are all greatly out of sync,
and (2) if the PTP or NTP process is stopped, the clocks start
drifting very quickly.

If it was isolated to one or two servers, I'd dismiss the issue.  I
also had this problem under CentOS 4.

I suspect something is mis-configured, because I can't imagine the
hardware clock on ALL these servers is *that* bad.

Anyone else dealt with anything similar?

Thanks!
Matt