Simplify the entire phone wiring in your house to an absolute minimum.
Even sockets with no phones plugged in can cause problems with your ADSL, so disconnect them if you can.
Test your router/modem with your pc on the first socket in your house (where the phone line comes into your dwelling) do this with all phones and internal wiring disconnected. (To disconnect internal wiring to test, remove the lower half of the NTE (bt main socket) and plug in on the test socket in behind) this puts you on the end of the BT network.
If you can get a connection at the test socket but nowhere else on your internal wiring, you have a problem with that wiring.
The number of people I have visited that tell me 'I have no problems with my wiring' only for me to see trapped wiring under doors, carpets and other high traffic areas, wires that have been stretched to hell to reach pc's/router/phones. Damp sockets, damaged internal wiring, ancient phones, incorrectly plugged in wiring/old modem cables/modem cables plugged into network ports in routers.
All these add up to YOUR RESPONSIBILITY.
CHECK, CHECK and CHECK again for all of these problems.
Any of these issues can and will cause you to have broadband problems.