F-150 engine races/revs while stopped at light

Dad's 2008 F150 small 8 cylinder has an unusual problem. The engine revs up on its own periodically when stopped at a light. Moving shift to neutral stops the racing. Dealer found nothing wrong and suggested floor mats are hitting gas pedal but that is not the case. ANy ideas what could be causing this?

I don't know anything about the 2008 F-150, but just in general rises in engine RPMs are caused by one (or more) of the following:

- Pressing on the gas pedal.
- More air is getting into the engine.
- More fuel is getting into the engine.
- A load on the engine turned off (A/C compressor, etc.).
- Cruise Control module is malfunctioning.
- Fuel (gasoline) is contaminated with a hotter burning fuel (alcohol, nitro-methane, etc.) causing the cylinder pressure to go up on the power stroke).

How high up are the RPMs going from normal?

trottle position sensors have been known to cause things like that i take it there are no eng light or codes