Frequent disconnects

I have had a slow but 100% stable connection for years, lately syncing at 1824.
But since last Monday (30/12), my connection started to drop many times per day, and syncing at much slower speeds most of the time.
I haven't touched anything at all in my setup, but looking at the line data in my Zen panel, I can see the run mode changed from Interleaved (as it was since 2008) to Fast, in the morning of 30/12, exactly when all the problems started.

Can be this the cause of the frequent disconnects and slow speeds, and why it had suddenly changed?

It is possible, and then the DLM should compensate for the drops and enable interleaving again.

Your best bet at this point will be to go through the base diagnostics to try and find what would cause the need for interleaving if it is anything more than distance.

Given the general weather recently, try a Quiet Line Test, to ensure that your phone line has not suffered.

Costs nothing except some time; and puts you in a stronger position if you have to call in the services of your phone provider.