Measure how much volatge is the alternator pulling
I had a similar problem this winter I did a couple of searches and most of the time after 2 regulator replacements the alternator is dead
I did replace my voltage regulator and it didn't fixed my problems. Now i have a new alternator
If i remember right when the engine is reving the amp should read between 12 and 15 and should be able to hold something higher then 13.
To be sure of the measuremnts do the test on a working car/alternator..