what is your internal wiring set up - can you simplify eveything and just connect direct to main incoming line box. Have you changed any filters/ tried a new filter?
As KW says though if line is not looking good suggests infrastructure issue outside of house
Yep, good point. Make sure the problem isn't in your property.
Most builders haven't got a clue how to wire phone extensions properly, so the best thing to do, IMHO, is fit a filtered faceplate to your master socket and separate the ADSL off there, so the extension wiring is isolated from it.
But, before you do that, remove the faceplate, check that all extension wiring is connected to the removable part of the faceplate and plug your microfilter and router directly into the test socket that is revealed by removing the faceplate. See if that helps matters. If not, it's your line, unfortunately, but if there is a difference then you can improve matters yourself.