tachometer question

["sujit roy" <sujitroy@hotmail.com>]

The tachometer needle on my '71 Stag fluctates while driving at a 
constant speeds. Any clues?  Some people have mentioned the voltage 
regulator, or it's the meter itself that is shot.  Is there any way I 
can test any of these parts without having to buy some from a 
specailized shop?

Is the voltage regular shared with another Triumph?  My local triumph 
grave yard has a load of Spitfires in.

Sujit