[c-nsp] NTP problems

[Roy]

I have been given responsibility for a bunch of remote Ciscos.  I have 
been steadily going around and adding some basic reconfiguration changes 
like logging and clock sync.  I have one troubled box.

If I add a new NTP server to the router then it seems to contact all of 
the servers and sync up.  However the status shows that the offset to 
"real" time just keeps increasing.  Example:

Clock is synchronized, stratum 2, reference is 207.200.81.113
nominal freq is 250.0000 Hz, actual freq is 250.0000 Hz, precision is 2**18
reference time is CB90F387.4268E82C (08:19:35.259 PDT Sun Mar 23 2008)
clock offset is -25407.1006 msec, root delay is 14.57 msec
root dispersion is 30283.54 msec, peer dispersion is 4876.43 msec

After a while, it just seems to stop contacting all the configured servers.
NTP debug shows

.Mar 23 08:21:16: NTP: packet from 207.200.81.113 failed validity tests 0A
.Mar 23 08:21:16: Bogus Packet received
.Mar 23 08:21:16: Peer/Server delay/dispersion boundary check failed

The unit is a Cisco 1721 with C1700 Software (C1700-K9O3SY7-M), Version 
12.3(18).

Any ideas would be appreciated.

Roy