99 rodeo "jumping" tachometer

I have a 99 rodeo 4x2, auto trans, 98k miles. Occasionally when in P or N, the engine revs up from 1000k rpm to 2k over and over again. it normally idles at 1k. when in gear it seems fine. Any ideas as to what's going on?

most likely a faulting intake manifold gasket. see the faq's

There was something else known to cause jumping idle as well but for the life of me I cant remember what it was. Idle air control maybe?