95 tracker Auto Transmission won't shift while cold

My 95 Tracker will not shift up gears while the car is cold. I have to let it run for a few minutes before getting on the road or it won't get over 25 mph. Once the car is warm, it shifts just fine. I have had the transmssion rebuilt. It helped for about a week. Now i am back where I started. The transmission guy can't figure what is wrong.