If it is indeed correlated closely with the Entanet node loading, then you have answered your own question.
Remember the days of BT Centrals? An Entanet node is a very similar thing, but less easy to manage.
It is in fact one of 20 BT Wholesale nodes, and at each node Entanet have an MSIL, which is the 21CN equivalent of a Central. On that they rent a certain maximum throughput level, but whereas on a Central the absolute limit was 622Mbps, an MSIL is 1 (or I expect now available 10) Gbps. As there are 20 nodes, that is a lot of potential throughput, but unfortunately your connection is hard-wired to the MSIL.
If the load on the node exceeds the rented amount the BTW allow it to go through, as long as it is below the maximum possible, but IIRC above 105% of rented the cost goes up dramatically.
Quite how Enta are handling this problem these days I don't know, as I understand the MSIL "ALT" was discontinued long ago. Perhaps BTW have introduced a "hard" cap to solve it, or perhaps your node is at the full 1Gpbs rental and overloaded.
But it certainly sounds as though that is where the congestion is, at the Entanet MSIL.
(That sounds a bit garbled to me, as dealing with uncertainties, but I hope it is understandable).
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