Time synchronization problems in OSX Mavericks (V10.9.2, latest patch)

We have set the Date&Time preference to pool.ntp.org.

The problem is, that during the day the mac server runs behind the correct time. So we have some problems between the serveral mac servers and their installed services. The servers do run all the time. No sleep options are activated.

We have tried to do it with some terminal commands like:

ntpdate -b pool.ntp.org but without success. We got the error:

3 Apr 08:24:27 ntpdate[23520]: the NTP socket is in use, exiting

How can we set the Date&Time preference in OSX Mavericks, that the time and date is always in synch with e.g. pool.ntp.org.

Thanks for your tipp in advance

Ritzo