was driving 70 mph and suddenly my car started smoking and rpm went high but speed dropped to 40 mph. It would not speed up at all. It did this 2 or 3 times, but once I got off the interstate it started opperating fine. The mechanic I took it to said that the transmission fluid was very dark. He even said that he thought the transmission fluid boiled over. All he knew to do after running several tests, was to change the fluid. He did test drive it, but still doesn't know what caused the car to do what it did or if it will happen again. Any answers to what might of caused this, and should i be concerned with it happening again soon.