Rev increasing while Gear in Neutral or Parking?

Hi, my 97 Ford Escort sometimes has this strange problem. When gear in N or P, the rev keeps on increasing, feels exactly like there was a foot on the gas pedal. So I have to switch off the engine quickly. This does not happen all the time, but when it happens, it really worries me. Could anyone tell me what this is and what I should do, please? Thanks!

Welcome to AF!!

This could be two items. 1. It could be a vacuum leak allowing air into the engine that is not controlled by the mass airflow sensor. 2. The throttle position sensor could be going bad and causing the engine to rev due to bad signals going to the computer. Check for the vacuum leak first and then the throttle position sensor.

Idle Control Valve?

Sounds like the idle air control valve to me... the small silver cylinder on top of the intake.