Check with BSNL Braodband Customer care and IF there is a heavy rain or water logging in your area then its not a big deal, you will have to go with that only.(Generally BSNL guys will change your wires and the problem will be solve as old cable slow down speed because of various reasons).
If this is the problem and you are not getting solution , Switch too Airtel.. it is sometimes better then BSNL and very good customer service. By the way i use BSNL Braodband 256 kb (unlimited) and i m more happy then who are having 2 mbps speed broadband.....