We have a 1998 Mercury Mystique. It runs great until you get up to about 65 mph. Then the car starts like almost stalls. It jerks real hard for a second then is fine. It usually happens going up hills if it needs to shift. I don't know what is wrong with it. I have had several people look at it and know one can tell me what is wrong with it. It has about 168,000 miles on it. Please Help