I have that issue as well. New V-reg a few years back fixed the bounciness in the needle but occasionally it will just die and a hard tap on the glass with my finger brings it back to life (half the time).
Most reading I have done suggests pushing tach out (into cabin) from inside the front trunk - I think you can snake your hand in by the brake booster. Clean and/or re-seat the connections.
The transmission signal generator unit is a puck type thing on the side of the trans. Not sure if that would be the issue (or even a culprit) since you (we) can tap on the tach and make things change.
Here is a link about cold solder joints inside the tach itself.
http://forums.pelicanparts.com/porsche-911-technical-forum/366603-tach-dies-when-hot-why.html