Loss of upstream speed.

This has happened a couple of time and am at a loss to understand why.

Recently, I had a minor issue with teh connection and BT resolved it however my 3dB downstram margin disappeared and I resynced with 6dB and 63Mbps downstream. Upstream had a 6.2 margin and a 19.8 Mbps sync. Speed tests gave an upstream of around 18Mbps - all fine.

Then, after a few days a full resync at 5dB occurs and downstream increases to around 68Mbps whilst Upstream drops, then a few days later another full resync with Down at 4.3 dB and 75Mbps however Upstream retains the 6.2 Margin (varies occasionally by +/- 0.1) but dropped to 17.3 Mbps and I fully expect tht when Down resyncs at 3dB and I see a 78 ish sync, Upstream will drop to 16.5 to 16.8 Mbps.

Why, when the margin remains the same and ine has not changed is there a major drop off in the Upstream speed as downstream increase? There are no shared tones, guards are in place ...

It was similar to this earlier in the year when there was a previous problem.