10.0.243.000 XPSP3/32

I can repeatedly show that when ZA has been running for a few days it causes access to NTP servers to degrade until eventually they are inaccessible.

I can't search for this topic in the forums Search as NTP is too short. I don't know if this is a specific NTP protocol bug (UDP 123), or just a symptom of a more general issue. Other network access seems to work OK (browsing for instance).

Basically I log the success of an SNTP client (based on a 2008 C++ example by Naughter) at accessing an NTP server on my ISP's local net (so on their local net separated from me by the ADSL link).

Initially every 10second access is successful, then after a couple of days it starts to fail until after a couple more days it is virtually inaccessible.
If I then restart (or just stop) ZA, I get the access back, and the pattern repeats.

And I believe this problem has existed for quite a while (months/years), it is only recently I have spotted the connection.

Please advise how I get this investigated and fixed. And any workarounds.