When I coast to a stop, a couple seconds after I put it in neutral, the RPM's shoot up to about 1700 RPM and then suddenly drop to like 200-300 RPM. Once the car even stalled. The only way to prevent it is to give it some gas. What could be causing this>?

 

Might be the idle air control is clogged. Also look for any intake manifold leaks. Throwin any cels?