I was having a problem with my internet recently. I run ATT DSL Extreme 6.0 with a Motorola modem and Apple Airport router.
It finally had slowed down to next to nothing so I got on the phone with them.
We went through a ton of settings, bypassed the Airport to make sure that was not the problem.
Finally got someone from line support on the phone and figure out it was a bad filter on the phone line. Once it was removed my speed jumped from under 1 Mbps to over 5.5.
