OK... so Im usually pretty good with cars but Im havin problems right now... hopefully someone out there can help me out with this one...
1999 Bonneville Supercharged 3800 series SSEi......135000 miles....

Alternator went out completly! So I changed it not a big deal... well now when you start it... it revs really high at about 2000 rpms then drops below 1000 rpms and idles there... like its gonna stall out but it doesnt... well while its doing all this my needles on all my gauges jump around like they are "dancing" lol... at least its running now since I changed the alternator... I just did that today... well I figured my idle just needed turned up or down or just needed to be played with so I was gonna work on gettin that taken care of this week... but I was driving it today and started smellin a sulfur/rotten egg smell... its not comin from the exhaust system so I know its not my converter.. but its coming from around the battery... so I think I might just try changing the battery.. but will that take care of the jumping of the needles... I dont kno much about this... so any help or suggestions are welcome!

Thanks!

Check all your vac lines. Your MAP is right next to the alternator.

There is a T to the right of the alt and map sensors like Dan mentioned behind the alt. (all orientations from facing alt).

Another possibility is the battery cables are loose or corroded. Pull them and the boots to double check. B'ville have a knack for corrosion based issues.