Perhap @JohnC2 can look at the performance monitors, if he still has access to them. Electrical interference would cause CRC errors, which would impact on throughput without affecting actual line speed.
CRC errors cannot be seen on the latest home hubs, but used to be visible from the BT end using Whoosh, but that was before I retired, so I expect things have changed now.
This can be worse when it gets dark, as you can get interference on the line from things like streetlights. plasma TVs, and light dimmers.
Hi @rf82 thanks for checking that and getting back to me. I've closed the case as resolved now and if you need any further help in the future do pop back to the community forum and we will be happy to help.