What's making my car's rpm's jump all over?

I have a 96 Monte Carlo (Chevy)...and lately it's been slipping or sputtering while running, for example, i'll be going 65mph on the freeway when all of sudden my rpms jump from 2 to 3... what might cause that? It only does it at random too... sometimes it will do it , others it won't..?? any ideas??