my car revs up on it's own

What is your question?

My car has a tendency to rev up on it's own..most of the time right after I've filled up the tank, or when I'm low on gas. Sometimes it stalls the car completely, but most of the time I put it in gear immediately so it doesn't stall..After I got the car I had the problem, and some sort of sensor was cleaned..but than a few weeks later it did it again..what could this possibly be, and how expensive might it be?