No Crank, No Lights, No Click, Battery Not Dead?

Over the past week I've been having a very strange problem with my electrical on my 72 Dart (225 slant). First happened when I was over at someone's house at night. Went I went back to my car and opened the door, the dome light didn't come on. Weird. I turn the key and there's no E-brake or oil dummy light, no 'click' of the starter, nothing. Had to be driven home. Friend takes the battery the next morning and recharges it at the auto store. They said it only took about 10 minutes to fully charge. Hook it back up, starts fine, I drive home.

Next time I got off of a short shift at work (4-5 hours) after driving it all day. I go and start it, same thing. No lights, no click, no crank. Jumping the battery did absolutely nothing. Showed it had about 12.9 volts with key in the 'on' position. Left it at work until the next morning. Again, disconnected the battery cables, saw the POS+ at the starter was loose and tightened it. Reconnected and started it up. I have not checked cranking volts when it actually 'comes back' alive.

The pattern continues. After about 5 hours (I timed it) the car acts as if I left the headlights on full blast for an entire day. Even after driving it for a couple hours throughout the day it will simply cut off. I have tried scrubbing the terminals and clamps and I am still having issues. It seems as though if I completely disconnect the battery for a few minutes and then reconnect it, the lights come back on. Engine cranks good, idles and drives like nothing happens.

More info: The ammeter seems to be recharging pretty harshly whenever I start up the car, pointing towards 30+ for a couple of seconds before dropping back down to neutral. Before it 'died' on me today, it took quite a bit of cranking to get her started, even though I only left the car for probably half an hour at most. The battery is a NAPA battery and dated June of 2013. Alternator is a newer 60 amp one from NAPA, and I have a 95 Dakota mini-starter. I have tried checking for any lights that would be on but can't find any. Only light dead on the car is the little light for the trunk. I have no idea what is causing this...any info would be appreciated. It's late at night as I write this so I can do more tomorrow.