Originally Posted by Stratman50th
I wonder if keeping it in a single gear will stop the shifting.
The heat is generated by the shearing of the fluid in the torque converter until it is locked which will only happen at cruising speeds, and in certain gears. I don't know off the top of my head, but I'm sure it will be in one of the 10 CC manuals.
Changing gears itself does not generate an appreciable amount of heat.
As stated above, check the fluid level via your selector, and/or see an Allison approved service center. If the level is correct, it might be possible that you have some sort of blockage that is impeding fluid flow; filter, hose, fitting.