What kind of speeds are you getting? Go to:
www.dslreports.com and run a couple speed tests. That will help determine what speed you are at. You should find out how far away from one of the central offices. If you're near the distance limit of 1.8KM (I believe) then you will see slowness.
Also how old is phone wiring where you are? Are you using an extension cord to plug the DSL modem into the phone jack (i.e. you need more then the supplied cable to connect the modem to the phone jack.
Sean