I have a 1999 Boneville SE. Not supercharged. It runs great other than rough idle while in park. I might not even call it rough, just not as smooth as I think it should be. RPM'* are about 800 while in park. If I shift into gear with foot on break, it idle'* just fine. Love to hear your ideas.

Bonnie_99

Check your EGR and EGR tube, mainly the one that goes from the EGR to the lower intake.