engine

I have a 1998 bucik park ave with app 120,000 miles on it, i recently had a new transmission put in and since i got the car back i noticed that the rpm will drop below 1 and kind of seize the car for a second everytime i am staying still at a light or stop sign. I took it to mechanic and they noticed that when you take off the engine oil cap there is a vacumm effect which should not be happening, when you remove the cap the rpm drops on the car also, is this a vacumm leak, the mechanic was bewildered and had no answer? also the car is making a slight squeakign noise from underneath?

denise