97 Ford F250 diesel 7.3 Transmission after about 100 miles overdrive light blinks and the low gear change occurs at higher than normal rpms. After a day or two the tranmission will reset and it drives normally. This has occurred over the last year or two. Is this a sensor problem or do I need a new transmission? Truck has 140k on transmission and has been only a city pickup...

 

It's probably the vehicle speed sensor. The way to know instead of guessing is to get the codes read. The places that read codes free won't be able to read the code that made the OD light flash.