NT Machines: System time problems

Help!! We have two NT4.0/SP6 workstations that constantly display the incorrect time. Their time can be corrected, and then 20 minutes later… they are off again. One of the servers advances the time ‘forward’ fairly rapidly. The other server laggs behind fairly rapidly. The servers are not on a domain. They are responsible for their own time. Both are DNS servers, one external (DMZ), the other internal (DNS,DHCP). Both have Norton Anti-virus installed. No software/ hardware changes have been made. Have googled the web for solutions, none found. Problem is about two weeks old. Need help fast. Thanks.