I took a 5 hour trip (one way) from Charlotte NC to Macon GA and all was fine going. But coming back on the interstate, as I would encounter a slight up hill grade my 850 N/A with 105,000 miles would start to buck...going in and out of overdrive. I would slow and move the shift lever to "3" from "D" and ride a while, then shift back. It might ride ok for an hour, then sart acting up again. I got home ok, but today my wife says the car is driving horrible and she worries about driving a few miles to get the kids from school. Any suggestions? Last year (20k ago) I flushed the tranny at the dealer. About the same time I replaced the rotor and cap, but still have original plug wires and plugs. Could it be the Throttle Position Switch? Or some time of speed sensor?