Sounds familiar.  I have been a thorn in my DSL provider side for years.  I could write a book on all our problems along with the trouble shooting it took to determine it was my providers problem.  Their answer was "nobody else is complaining so we won't do anything".  We finally were able to get a stable signal - long story on how we accomplished that, but I noticed that our bandwidth kept collapsing.  The solution is to adjust the QOS settings in the modem to help keep the outbound traffic packets from collapsing the inbound bandwidth.  Hopefully you will have better luck than me at your provider, but the equipment they use had no QOS settings according to their tech.  They consulted the tech for manufacturing company and they agreed that residential routers did not have that option in their firmware.  We ended up buying a commercial grade router with tomato on our own, and I was able to make the connection stable with its settings.  I offered to give our DSL provider the solution, but...."nobody is complaining".  I would switch in a heartbeat if we had a better option.

Good luck