2002 Oldsmobile Bravada Engine reving up and down while par

Engine Performance problem
2002 Oldsmobile Bravada Automatic 27900 miles

my engine, when I come to a complete stop, the RPM shows (and I can hear it) the engine revs up and down and down almost to the point where it sounds like its going to die. I just had it checked for everything recently and the mechanics say that there is nothing wrong. Why is my car reving up and down while I stopped?