97 4.6L- At times, my bird will go into "autopilot" mode: When I take my foot off of the gas, the engine will either continue to maintain the current acceleration rate, or occasionally when in 2nd or 3rd, will actually start speeding up. Braking to slow down becomes ALOT harder and gives me a front end shudder. When coming to a stop, after gearing down to 1st, RPM will jump up to 1000, requiring alot of force on the brake to keep the car stopped. Finally, when stopping to turn the motor off, the RPM will jump up to 1500-2000 when the shifter is moved to Park position.
This happens occasionally to frequently, sometimes while in the city and other times on the highway- there is absolutely no pattern as to the frequency, and there is no check engine light.

 

It may be a sticking throttle.
Check the throttle cable for operation, clean and lube it, if needed replace it. Clean the throttle body with throttle body cleaner. Make sure the throttle return springs are good.

While you are there look for vacuum leaks as well.