strange alternator behavior

I have a '92 Daihatsu Rocky, but I think this is a general alternator behavior question...
Background: vehicle has had a bad battery, recently sat for a LONG time due to a leaky radiator, got that fixed, replaced the battery, and now, intermittently, the alternator puts out 20V (verified with digital multi-meter at battery). I tried to start the car again in the morning and the volt gauge showed a normal range (again verified with multi-meter). I took it for a drive and it will intermittently peg the Volt meter, but if I turn on the blower fan, the voltage goes down to 15ish volts (it's only a 50amp alternator).

Is it possible that there is an intermittent connection for the "field coil" (I think that's what it's called). Is that even possible? What if the high tension positive cable at the battery didn't have a good connection? Could that cause the voltage to spike?

I think it's time to replace the alternator, but I'd rather not spend $150 bucks and have it be something else.

Any ideas? I am going to mess with the connections at the battery tonight with the vehicle idling and a voltmeter at the battery...

Seriously unless you absolutely need to keep the alternator light working just get a single wire GM 63 amp from Napa with a lifetime warranty for 44$ with a core turn. Alternators tend to fail catastrophically but I have seen generators fail in the manner you are talking about.

In reply to wearymicrobe:

Parts for this thing are getting harder and harder to source in the US, so that might be an option, but then I have the possible pain with mounting it. But definitely something to think about.

In reply to jstecz:

http://www.chargerr.com/Alternators/ALT.HTM

Find out which is the "sense" or "monitor" wire on the alternator and see what kind of voltage you have there vs. the + battery post. Any real voltage drop is a problem. It sounds like yours is completely missing.