AeroNUTical

99 GT auto with 121K. Having an issue while shifting from park into gear (R or D). Car will idle anywhere between 600-900, when shift into gear the tach drops near 500 and what I suspect is that the computer recognizes this and adjusts, causing a burst (add gas)in RPM on the tach. The car lunges a little even with brakes on. I'm just curious if this is a fuel issue (injectors, etc) or a vacuum leak? Any suggestions would be helpful.

Thanks