This may seem strange but is your dsl modem with in 6 ft of your microwave, Floresent lights, cell phones,Wireless routers , Or 3ft from any other electical device try wraping your modem in Window screen(Metal) see if that helps if it does i would move the modem. Also Call Your ISP and ask them to check your Port Rates and SNR.
-J
P.S. DSL Has 10% overhead for ATM frames and about 3% for TCP/IP