keep time in sync?

Hi all, I have troubles with a server getting time right. On boot the time is synchronized with the ntp-server of my ISP (xntpd is started at boot). After that nothing seems to be done any more. This machine has a clock, that runs about half an hour behind in 24 hours, so timesync is something, that could be usefull :-) What do I miss?? Leen.