It sounds like you have the same problem I had with my 99. My transmission was hunting in & out of overdrive at @ 40 MPH it turned out to be a defective ECM which was covered under the 8 year or 80,000 mile EPA emissions law. It took almost 3 months to find in the end. I had to take my truck back to the dealer 5 times before they agreed that the ECM was the problem. All I can say is hang in and let the dealer do what he has to do to find the problem. You can get so good information on the DTT transmission web site on this type of problem. Best of luck.