My car is a 4 cyl with automatic, 150,000 miles. For the last three years my transmission shifts normal when the tranny oil is cold. When the oil heats up the shifts are not crisp, there is slippage and I have to go easy on the throttle. Needless to say, I don't drive it very far, only about 8 to 12 mile round trips or some cooling off time after going one way before the return trip. What do you guys think the problem may be???