i had this exact same problem before going to broad band. the guy at our local computer shop said to phone our telephone provider and tell them to turn up the gain. i believe the problem is caused by other people having broadband on the same telephone line and it steels all the power.
another thing is checking the wire to see if it's knotted, or if anythings lying on it. when i had dial up, it used to disconnect when someone trod on the wire.