Another vote for the torque convertor locking and unlocking. In mine it was causing a 200-300 surge when cruising at a light throttle. Much more pronounced gonig up a hill with little throttle. To fix mine I disconnected the battery and put the two terminals together to "reset" the computers. Did that about 4 months ago and I haven't had a problem since. It's free to do so you could give it a try.