I have a 1995 Volvo 960 with about 160,000 miles on it. When driving on the highway the car has a bucking sensation. This only happens like 3 or 4 times during an hour drive. When I lift off the throttle a little bit to slow down is when it happens. When the throttle is pressed down to maintain speed it does not happen. It feels like there is some sort of hesitation for a second when the throttle is let out a little bit. The car sort of bucks back and forth, it is very very subtle. Is this related to the transmission or MAF or ETM? The car starts has no problem starting in any temperature. No strange noises or anything from the engine.

I know that the car does have a vacuum leak somewhere because the AC changes venting positions when the throttle is pressed. I have read about this on other forums. When you accelerate, the AC goes from the dash vents to the floor vents and then back when the RPM goes back down.

Does my bucking problem have to do wit the vacuum leak?

Thanks for the help.