I can't see anything obviously wrong from this graph. It's late at night so I would expect a slightly lower noise margin (I assume your target is 6dB). However your sync speeds are varying a lot. The only thing I can suggest is to keep routerstats running all the time and see if you get disconnections/resyncs.
Can you confirm that you have your modem or router connected to the test socket on your BT master socket? Oldjim posted a photo in case you're not sure where this is.
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