Tach tantrum!

[jrjajs]

Took the 94 s4 to work today. On the ride in all was good and normal. When I left this evening the tach immediately pegged itself when I turned the engine over. Also a sort of very low rpm on the engine and a stall after about 20 seconds.
Turned it over again and the tach pegged itself again. The engine ran normal and all seemed fine except the tachometer. As I drove home it kind of hunted around between being pegged and sitting at what would be 4500/5000. Almost seemed to be using that as a idle rpm point.

Any help would be greatly appreciated.

Thanks!!!!