dodge truck transmission shifting problem

This is my first forum. Can someone tell me why my 1998 Dodge truck with 90000 miles shifts in and out of overdrive when I drive the freeway? It drives me crazy and no mechanics can reproduce the problem. It is a used truck that I am rebuilding and don't want to pull and replace the transmission unless I have to. Thanks

its called a tranny surge. mine did it for 20k miles. believe it or not, go buy a throttle position sensor, its about 40 bucks and its on the drivers side of the throttle body. theres nothing wrong with your tranny. the sensor devolops a weak spot and it causes the ecm to think your moving the throttle enough to pull you out of OD. please post your results, for it helps alot of people. BTW my tranny hasnt dropped out of OD on the highway since replacing it. its not just OD, if you turn your OD off, it will still surge out of drive.