The Fritzbox is now in, it took quite some effort to get it up and running (a neighbour helped in the end) but we're there.

Unfortunately it's not changed the symptoms at all and I've been cut off twice already while checking some things remotely. The usual power cycle brings things back again.

Actually the neighbour mentioned that the whole area had "old cabling" which caused poor internet for a lot of people.

During my 3 weeks there I didn't see the DSL drop once but I needed to reset the connection as many as 50 or 60 times. The more internet traffic the quicker it would die. Although it was easier to hit the "neustart" button the internet could be recovered by disconnecting the DSL cable and reconnecting it.

So I suspect the cabling is fine and you've got a problem upstream of the DSLAM.

Upon failure the PPPoE cannot recover and times out endlessly despite the DSL sync being OK.

We have done 3 router swaps now and the line has been checked by one of your engineers.

The problem is clearly not related to the line or customer side equipment. It's time for you to escalate or there will be no further progress from here.