Dimension 4 no longer crashing but stops updating after a 6 hours or so...
***** RESOLVED (BUT NOT FIXED) BY MAKING MY OWN TIME SERVER VIA A GPS MODULE WHICH ALL PCs ON THE NETWORK CAN POINT TO *******
It seems that the NTP servers watch for requests and if they continue too often then they block you...
As no-one else has this problem then it must be a problem specific to this setup.
Will have to think of a time sync solution that doesn't involve NTP servers - they don't seem to like me...