Have a 2000 Durango, 4x4, 4.7, 74K . During a 400 mile trip while traveling about 80 MPH the truck seemed to kick out of OD and then go back in. This occured while under a light load ( small hill ). This has never happened before. About 200 Miles later ( almost home ) the same thing happened again, only this time it happened several times while attempting to accelerate up the hill. From this point on everytime I came to a hill and tried to accelerate up it the truck would continue shifting in and out of OD until I let off of the gas. As long as I stayed at a steady speed it went fine. For the next hill I switched the OD off, the tach stayed about 3K, speed about 75 MPH and went up hill with no problem. Then I discovered the MIL was lit. When I got into town starting from stopped position, accelerating from light to light and crusing 55MPH and under the truck ran fine.( I had turned the OD switch back on ) I got home and pulled the code ( P0121 ) I don't know if the problem is a result of the code or the code is a result of the problem. I checked the books and on the Net but there is not much info as to what causes this code. I got the generic definition ( TPS/MAP Range/Performance Problem ) Have checked the TPS, smooth pursuit from idle to WOT, and I do have 5 volts from the harness. I don't know how to check the MAP it doesn't have a vacume line it is bolted to the intake. MAP also has 5 volts at harness. Any help would be greatly appreciated I don't want to go to the dealer.