2001 Chevy Cavalier Accelator problem

I rescently started experiencing proble, where when I go to accellorate the gas pedal is hard to push and the car is slow to accellorate. It has no problem shifting and sometimes it runs fine but usually after sriving it for a short period of time the gas pedal gets hard to push. Sometimes I can park it and it smells like something is burning like oil and when getting gas there is usually air that comes out when removing the gas cap but not now. Could this be the throttle sensor or just the throttle cable? The shop said the cable was wrapped around something and rerouted it and it ran fine for a couple of hrs then it started again. If the sensor how much should this cost?