That sounds like it'd be a nightmare to get investigated
Intermittent faults are indeed the hardest to troubleshoot.
A change in attenuation is something that should never happen ... but when it does, it also seems to be an indicator of one of these tricky cases.
The easiest thing for you to do yourself is to take the faceplate off, and run the modem through a rat-tail filter, and just leave it like that to see if you see the same behaviour (now, or at some point in the future).
I'm happy with the current speeds, just hoping it remains that way. Would you suggest I leave a phone attached to the line? It'll be annoying but possible (the master socket is in my 9 year old sons room, I didn't really want a phone in there! )
The thing that might have "fixed" things temporarily is the current that was drawn by the phone (after a few goes); this current can have "burnt through" a layer of oxidation, to make a proper circuit again ... and if so, the current is called a "whetting current".
It is never more than a temporary fix, as only a small amount of the oxidation will have been removed, and it will soon build up again, so cause problems again.
When it does go wrong, then you might need to use the phone again. But you don't need to leave it in place 24x7 ... only when you need it. Even if you choose to go off-hook every day, hoping to act as a preventative.