trans no go

Helped someone change the 4t60e in a 96 GP. The donor was another 96 with 31000 miles and a known working trans. It works great with no slipping and shifts good. The only problem is if it sits for 4 or 5 hours it wont move until the engine runs for about 5 minutes. I thought it might be an air lock but since its been driven for several days I dont know.
Thanks for any thoughts on this

If the tranny only has 31k on it I would do a full tranny flush with a new filter and pan gasket. That should take care of air locks and junk in the fluid. If it doesn't move after that try flashing the PCM or maybe even check the shift linkage but those are just wild guesses for some stupid problems I've heard of before. Good luck man.