Lincoln/Mercury Repair: 96 Town Car with only 50,000 mi, vacuum leak, town car

Question
 Go Patriots,
      and Help me Dinger,
            Just bought a nice Town Car.  Low miles and bless his heart the owner passed away.  the car sat for 7 or 8 months.  Lets say I am running 40 mph and a stop light is comming up. I let off the throttle and coast. The car wants to somehow PUSH and Lurch against the Downshift. It feels like it is wanting to accelerate on its own or something like that.  The family had it tuned up and installed a new shift mod. before selling so I am wondering if the problem was already there. It did not act up until AFTER I bought it of course. I really love the ride and I was hoping you could give me the answer. Thanks alot. The Pats kicked Butt today.

Answer
You know, they really proved themselves of being a contender this year with handing Chicago an upset. And last week, smoking Green Bay!

Well, Kev... Hard to determine exactly what is up with your car. Is there a check engine light on? I hope so, because that would make it easy. Now you stated they installed a shift mod. Did you mean a shift solenoid or a transmission module? Well, the more I think about this it leads me to believe there is a vacuum leak. I think when you let off the gas, the engine increases RPMs to compensate for the lack of vacuum. Making it feel like it's surging on. Now, finding a leak is no fun, I have had better dentist trips. The easiest way is to look and listen to a light hissing noise. You can also use a can of brake clean and spray it on a suspected spot. Let me know what you turn up...