Allison 5sp: trans goes into safety mode/drops to 3rd gear.

[fguerrina]

I had a chip put in. Truck performs relatively well. 300k miles. Accellerate to pass, get to about 70 or 75 mph accelerating and trans kicks to 3rd. Sometimes have to shut truck off and let sit for awhile to get trans to reset. Is this just high mileage. fluids are new.

 

[TheBac]

Moved into Trans Talk area.

 

[ALLY Fox]

It won't heal itself, it will only get worse plus you run the risk of plugging the radiator trans cooler inlet with clutch debris. You'll have to bust open the piggy bank and have it built. I always recommend Inglewood Transmission. http://inglewoodtransmission.com/jm1/
Mike can recommend a trusted shop near you if you are not near LA.