I've just fitted a Saas 52mm Tacho which works but needle bounces all over the place. Have tried the following. Made sure it was connected to negative side of coil, fitted a resistor between coil and gauge, checked and changed earth wire, setting on rear is set right. 4 6 8 Other suggestion is fitting a diode. Any ideas?

You'll need to install a diode between coil and gauge. I had the same problem.
http://www.thesamba.com/vw/forum/viewtopic.php?t=566900&highlight=diode 

Yea the diode is needed to allow residual current freewheel to earth

thanks. worked a treat. soldered a diode in and now tach works well. cheers