Don't have a clue what could have happened, but my DSL slowed way down on me about a week and a half ago, and it got trouble shot back to 2 components....my DSL modem was bad, and my 1751 router would not obtain anywhere near full speed (600Kbps - 2.5Mbps out of a 10Mbps Link). I pop my old linksys back on after replacing the modem and getting erratic good speeds to provider (5-8Mbps), but slow (1Mbps) to outside world, and all of a sudden I get the full 10Mbps link and everything works fine. Also my ping time between the two to the same server was constant 19 with linksys and 43 with cisco.
Haven't changed any config. The MTU's and everything is all the same, not to mention it was working just fine for me before. maybe WIC-1ENET card got damaged? Is there a way to tell without having a spare?