I have an '02 A6 Quattro 4.2L that has an acceleration problem. If I slow down at a stop sign and roll through without fully stopping when I try to accelerate sometimes it feels as though the traction control has kicked in and while I get high revs, it will not shift unless I take my foot off the accelerator and then press down again. Bad speed sensor? My mechanic has said it hasn't thrown an error code so what gives.