idle and oil pressure unison variations

I have a 1994 cavalier rs with a 3.1 multi-port fi v-6, i have a problem where the rpm gauge and oil pressure gage will fluctuate in unison in drive and reverse at idle speed. Also the car has been sputtering when driving at crusiing speeds, almost like it has a miss in the firing. i had my car in for a tune up at a local garage two weeks ago, and the problem has been present since recieving my car back from mentioned garage, i am wondering if anyone knows possibly what the problem could be, and if they could please let me know right away if they do know, cause it is really racking my brain