just replaced the cruise control amplifier on my 84 300 D turbo. now when the cruise is engaged and I drive down hill, the car seems to surge. it's as though it can't decide whether it's going downhill or needs to continue to accelerate. when it's going up hill it does fine, and pretty good on flat land (which we don't have much of here in East Texas. Is there some mechanism in the cruise control system that can tell if the car is going downhill and makes the cruise react accordingly? or is the amplifier bad?