STAS users randomly disconnecting

STAS users keep disconnecting and have done since I purchased the 2 XG230 units. I've had a support ticket open since Feb 2018.

All tests in STAS (WMI, Reg, Agent, Collector) show as successful.

Live users appear in STAS and then randomly drop off, anywhere between 4min-4hrs.

Recently added another domain controller running the Sophos STAS agent. Any clients authenticated against that DC, in 'Show Live Users', display the XG's ip address?!

Then, suddenly appear again (twice) with the correct IP address and then drop off.

Any advise would be greatly appreciated.