Speed Control Springs problems of the 1996 Mercury Mystique

The throttle freezes. At times the throttle feels like it is dragging causing you to press harder than needed to accelerate, then the car springs forward. Some times when you push lightly on the accelerator it starts out into the traffic lane then the throttle freezes. No matter how hard you push it will not budge, then suddenly it will free up and work fine for days maybe then it will do it again. Very unsafe. We have had a couple of close calls. Ford- Mercury are really dragging their heels. They have charged me over $100 on one visit for this problem and fixed nothing. Thanks you for your help, dan.