Trasmission ???

Wierd problem. '97 Sable Wagon. My transmission slips, only until warmed up. On summer days it is almost undetectable, but during the winter it takes a few miles or more to get smoothed out. Once it is warmed up, it runs like brand new. Never slips is 1st gear, only at shift points. Never heard of such a thing???? I've had trannies that slip, but they slip all the time. Any clues?? Thanks.

It maybe old tranny fluid being thickened by the cold and not flowing properly until it gets to the proper heat range.