1989 Buick Skylark car has to warm up before shifting into

Have this same problem. Did you ever get an answer to what causes this?

Ok, I can assume the transmissions have never been rebuilt since it is GM. (Fords would have gone through 5 units and chrysler 3 by 80K) Anyways, the reason why the unit has to get warm is that the seals are hard and the fluid goes by the seals since they cannot seal up the apply pistons inside the units. There r two ways to fix, one is to rebuilt the unit and have the unit get new o'ring seals, the next is to use "TRANS-X" in the fluid. This is a last ditch effort in getting the seals to seal. It swells and softens the seals, but can also do damage to the seals, so follow the directions to the letter! Good luck!