Just an update to this thread I posted a couple of months ago:
This problem was still ongoing and I had to regularly reboot the problem server.
I was going to take Forthill advice and do a rebuild/reinstall job on the server over the Christmas break.
However, last Thursday I had to rebuild a large number of my legacy machines because they weren't recognising
USB drives (thanks to Blackcat Spider software... don't get me started!!

).
Since these machines have been rebuilt, the DNS problem on the server has ceased.
Its been 7 days now and the event viewer hasn't shown a single error. At this stage I don't think
its a coincidence.
So if anyone else is having this problem, it might be worth thinking about systematically rebuilding stations
before embarking on major server surgery.