Accelerator temporarily won't Move.

1999 Toyota Camry. 120,000 miles. Four-cylinder engine. My parent's Camry has had an issue with the accelerator not moving when it is pressed. When the accelerator is pressed from a stopped position, sometimes the pedal will not move, as if it is stuck and the car won't move. After pressing it a few times, it will "unstick" and is then driveable. This doesn't happen all the time but when it does happen, it may happen several times in a trip. They went to the dealer and was told there may be dirt somewhere by the accelerator. I guess they cleaned it, but now the problem is happening again.

Have you heard of this before and do you have any suggestions?