How many other computers are on the network and what OS are they running?
You can set up a similar fudge time source on the client so when ntp does not like the server's time it will switch to local time and not stop running.
In addition you can manually adjust the clock to improve long term stability. The following link provides some details.
http://www.smartpixie.com/wiki/Tech/...lockGuide.html
How important is keeping accurate time between the server and client? GPS receivers are cheap and it is fairly simple to set up your own time source.