In my 99 CTD 4x4 with 144,000, when driving between 40-50 MPH the trans has started shifting up and down between 4th & OD. Under 43 no problem-solid 4th, over 50 no problem-solid OD. Truck has been babied since day one, always serviced, only towed about 6 times in 11 years. Iv'e heard about a switch on the outside of the trans whose contacts can get dirty. I'll check the contacts or swap out the switch, but it is possible it could be something bigger?

 

clean and check battery cables and ground connections

 

Ran a new ground wire from the TPS to the main ground under the dash and it cured the problem in one try. check www.suncoastconvertors.com and look for "24 valve APPS rewire ground instructions".