Tachometer sometimes works sometimes not. Usually reads correctly or 0. Rarely reads low. Could it be sender? Where is it? Could it be ground? Where is it?

Thanks

 

The tach gets it's signal from the DME, and the most important input for the DME on a 944 is the crank speed/angle, considering that your car probably won't even fire without a correct rpm signal., your problem is most likely the tach itself, or possibly the connections behind it. The entire panel isn't that hard to pull, let me know if you need further help.

If it's indeed your tach that's broken, you can replace your tach with a used one, or have it rebuilt. (Parts heaven is the only place I know that can arrange shipping you a rebuilt one).
Ahmet