Does moisture cause tarnish on phone copper wires and affect phone service (but not ADSL)?
Answers:
It's possible, but it sounds to me like you have a short somewhere that the repairman was too lazy to look for. DSL doesn't need a dial tone to work, since it is a digital signal, and it can be crossed and still be functional. Tarnished lines, however, would cause the DSL to quit before the phone. Sounds like a short. You know if you leave the phone off the hook for too long, it eventually goes to silence. A short would have the same effect as having the phone off the hook. Most likely, there's a wire sticking out too far from one of the terminals and touching the others. The reason it might get progressively worse, or affect more than one line, could be vibration or something, not really sure about that. Working in an apartment like that can be overwhelming for a repairman, so if he's lazy, he may not search for every terminal in the house. They're very simple, so you may want to check them yourself. Also, if you can follow the lines ANYWHERE, look and see what you can find. If all else fails, you will have to somehow rerun the wires, but goo luck with that in an apartment. You may even ask your superintendent if there's a chase somewhere that they could look into.
An easy way to pull new wiring would be to tie the new wires to the old ones, fold and tape the them, and pull them from each junction box. There shouldn't be too many junctions in an apartment, since they're wired the most simple way possible. Be careful pulling the wire (do it with all the wires at once) because it is a very small gauge and you may break them.
Other answers:
what doot said.
what doot said.