Last night the service guy from Telecom finally appeared, and in the end figured out that 2 cables have been jammed both in their box in the basement aswell as in the box in our room. Now for tech-savvy people it might sound reasonable, because jammed cables usually don't transmit data too well.
However, I fail to see how jammed cables are the reason, if - for weeks it works fine - but out of the blue it stops working properly, i.e. Sync-/DSL loss, with the connection coming back after a minute or two. I mean, if cables were really that jammed, it (technically) should not work at all. But since it worked sometimes, I still don't trust that matter. Or maybe cables unjammed on their own for no specific reason, just to go back to the jammed state hours/days/weeks later. The last person that had actually touched the cables before (and found them to be "ok") was this very same service technician. Maybe he went around our house and jammed the cables out of boredom just so he has to fix something. Or perhaps some neighbor (and the people that live here probably have even less a clue about telephone/internet stuff than myself and are all considerably older than I am) went around and jammed some cables for fun.
Could have been the rats too..oh wait, we don't have any. And not sure they'd particularly jam cables (rather than bite them through anyway).