At highway speeds my 1996 f250 diesel p/u is changing RPM. At 65 mph it normally runs around 2000 RPM. Periodically (every 2-5 min) it will jump 2-300 RPM, stay there for a few seconds and then go back down.

Had a similar problem one time where it did the same thing but only when I turned on the turn signal. Found it to be backfeeding to brake circuit. Changed out turn signal switch and fixed the problem.

Any guesses as to what's going on this time would be much appreciated.

 

Assuming it's an automatic trans, it sounds like the TC is unlocking. That could be caused by a number of things, but I'd bet it is either the TPS or the TCC solenoid beginning to fail.