[C320-list] alternator question

[dmcnamara3 at woh.rr.com]

Hello,
I have a 94 Catalina 320 (064) I have an issue that purplexes me.  When I bought the boat last year the alternator (original Lucas) was non functioning.  I purchased a new one and it worked one time.  I thought maybe I did something wrong like turn the key off before stopping the engine.  So I purchased another.  I installed it last weekend and it worked! had 14 volt at the alternator with a volt meter, the amp gage showed 14 volts, and the charge light went out, the rpm gage worked also. This past weekend when I started the engine, only 12 volts at the alternator, 12 volt at the meter, charge light on and no rpm gage.  I did not turn the key off before stopping the engine, I did not turn the master switch between batteries A and B while the engine was running.  I have cleaned and inspected all of the wires inside the pedestal and at the terminal block at the engine.  The large fuse on the ground side has also been checked.  Any help would be greatly appreciated.
thanks
Dan