1981 Ford Fairmont 1981 Ford Fairmont Charging systems prob

So, my battery dies while I'm driving my car. It's a brand new battery. I check the terminals and one is loose so I tighten one, replace the other.

The car runs a few days no problem, but then my battery dies again, but after I park. The first time it stalled, now it will run, but as soon as you turn it off the battery is dead.

I took the new battery and the alternator to the auto store to see if there's any problems. They say no, they both test good. I buy a voltage regulator thinking that must be the problem. I reinstall the old alternator and the new voltage regulator. The auto store charged the battery so the car fired right up. After a half hour of use, I park it and it happens again. No juice in the battery.

Is there anyway the alternator is bad, even though it test 13.8?

I was just wondering if there was any suggestions before I take it in. It's christmas time and I'd really like to save a buck.