It could be the piston seals in one of the drums has hardened over time. When this happens, they don't seal well when cold and stiff but can start working after they warm up and become more pliable. A line pressure and stall test would be recommended to diagnose the problem, but, if it is the seal or a problem with a clutch assembly, it will require teardown of the transmission to fix. At that point, it would make more sense to overhaul the transmission or replace it. Otherwise, you might try using a conditioner like Trans-X and keep driving it until you have to do something about it. The factory service manual will give you a symptom chart that will list all of the possible causes for your concern and the fixes in order of most likely to least likely.