99 Olds Aurora sputters when engine reaches 2,500RPM's to 2,700RPM's?

I recently took my 1999 Oldsmobile Aurora in becuase the Alternator went bad. When the shop I took it to replaced the alternator, they must've did something becuase now my car will sputter at 2,500RPM's - 2,700RPM's. Every other RPM range runs smooth as gl, only this very DISTINCT RPM range. I thought it must be a sensor, vaccuum or ECM module, but the diagnostic test they ran is not showing any codes. Again, car runs perfect, and this is not related to any paticular speed range, it's simply the RPM's of the engine that makes it sputter. Also, now when it sits and idles, it will go from about 500RPM's to 900RPMS and go up and down about every 2 to 3 seconds as if it is trying to find the right idle speed. What could have possibly happened and how can I check it?? I'm a moderate mechanic with pretty good working knowledge of this car, so any deeper troubleshooting methods are ok to p on.