I had similar problems with my RPMs both jumping and dropping.
When my RPMs were jumping i would be stopped and out of gear (manual tranny) and it would rev itself to 2000-2500 and for that i pulled out and cleaned all my fuel injectors, changed the spark plugs, and ran a bottle of sea foam through it. I also went in and made sure all the connections on my engine sensors were good.
For my RPMs dropping I would have to actually give it gas to keep the engine from dying when stopped and that was caused by a short in my front indicator lights. (gotta love Chrysler electronics!:icon_crazy
Other common problems that might cause this are a bad TPS (throttle position sensor) or CPS (crank position sensor) Do you have a check engine light?