[C320-list] Alternator issues

[dmcnamara3 at woh.rr.com]

Hello,
I had asked for assistance on this issue about a year ago, but my problem persists.  I have a 1994 Catalina 320 with a Perkins 30.  When I bought the boat 2 years ago the original alternator (Lucas 55 amp) was not functioning.  With the engine running,  there was no voltage output checked at the back of the alternator with a volt meter, and the alternator light was on and the tach was not functioning.  
- I went through and cleaned all of the electrical terminations on the terminal block. 
- Checked the large fuse on the back of the engine.
- Replaced the battery selector switch
- New batteries
- Replaced the old battery charger with a new one (also check polarity on the plug to the battery charger)
- Replaced the alternator with a new one
At this point I started the engine and the new alternator was putting out voltage (tach worked, charge light went out)
The next day alternator did not put out again,  replaced alternator with another new one.  Everything worked again until the next day, then nothing.
I'am not turning the battery switch off or turning the key off before stopping the engine.  I wait until the engine is totally stopped before turning the key off.
Using an Ohm meter set on the diode selection shows OL (positive lead on positive post of alternator) and 1.035 (negative lead on positive post of the alternator) If I read this correctly the diodes are not blown.
Sorry for such a long post, I thought I would try to solve this issue before I call in an electrician.
thanks
Dan