Charging issue.... I think

Well, my Dart has an issue with keeping the battery charged up.

Symptom: with a fresh battery, car starts and idles good. After ten minutes the idle goes to crap (it just dies). Try re-starting the car and it turns over like the battery is weak. Jump the battery with my truck and it turns over fine and fires right up. So, one would think that the alternator is junk. Took the alternator off and had it tested. It is fine. It even shows 13.8 volts at the battery when the car is running.
Tried a different battery and the results are the same.
I have a one wire alternator in the car (powermaster unit). I also have an electric fan, an MSD ignition and a Holley black electric fuel pump. Those are the only three aftermarket devices in the car.
Car was fine for two seasons like this. All I did was remove the heads for a port job and now I can't fix this problem. Any suggestions??