[C320-list] Tachometer antics

[Magnuson's Time A Weigh]

On my 1995 hull 205, I have 484 hours on my Perkins M30, my tachometer has 
started bouncing between 0 and what ever speed the engine is running. I just 
tried the following here in the slip:  @2500 rpm, tachometer solid - volt 
meter steady at about 14.8v
2000 rpm same,  1500 solid then bounces between 500 and 1500 while voltage 
flutters between 14.8v and 15.3v
1000 rpm tachometer bounces 0 to 1000, voltage flutters around the 14.8v.
With ignition key turned off, engine still runs, but tachometer stays solid, 
and volt meter flutters between 15v and 16v.
Turn key back on and the tachometer will bounce again.
I don't have an electrical schematic on board and being a Master of The 
Obvious, I assume it is an alternator, regulator, transducer, or wiring 
connection problem.
Will someone share their experience and solution.  This site is such a great 
resource!!!

Thanks,
Gary Magnuson
320 #205