Overdrive--8.1 Vortec-W-22-1000

Allison man---My coach runs and shifts normally until I am on the road for 30-90 minutes. Then it starts downshifting from overdrive to 4th and back up to O.D. very erratically. My only solution was to lock it out of overdrive by using the switch on the dash. So far the only answer is "to bring it in" but no-one really seems to have a clue.

Ever since new, this transmission, usually doesn't shift up to O.D. until I lift my foot from the throttle around 55-60mph. On one ocassion, I experienced the clunking and shuddering noise while putting it in reverse---but that only happened once and has never re-occurred.

Could this possibly be a heat related issue?

Thanks for any suggestions