For Steve Reeves:

Steve:  Intermittent problems can be extremely difficult to solve.  In your case, I would start by checking the alternator/regulator wiring, and especially the grounds.  How old is the regulator?  If the alternator is easily removed,  any electrical shop can check it out for you.  If the alternator checks out,  then the problem most likely is in the regulator.

Sizing an alternator is primarily a function of your steady state electrical load plus fudge factor for your intermittent loads.  Take 120 % of that total and  you should be pretty close.
Using a 100 amp alternator to service a 25 amp electrical load is overkill and just adds unnecessary weight and additional cost.

Hope this helps.

Best Regards,  Bill Hogarty