Did you try to change the splitter?
If your splitterbox (connected beetween the telephone socket and the DSL router / phone) is very old, and your isp is a good one, they'll usually give you a new one for free if you turn up with that in the store. Old splitters may show weird problems sometimes.
Also, did you check the telephone cable aswell? It may be broken. Do you have quality problems when on the phone?