Alive Dead Alive

I was driving my 72 swinger around 2 weeks ago and after about 30 minutes of driving I went into the store for a few things about 40 minutes later I went to start her up and she wouldn't go. After a few minutes of waiting and cursing she fired up and ran fine. About halfway home she died again as i was letting off of the gas for a stop sign. This time I could not get her going again. After a few "almost got it running" attempts I towed it home with the help of my brother. I was thinking Ballast resistor, because some times it would run while holding the key forward but then die when I let it go back to the run position. Today I got my new resistor and after a jump start from my cummins it was purring like usual. I assumed everything was okay. Boy was I wrong. After about 20 or so minutes of driving, I was turning into my neighborhood and as i was letting off of the gas the same thing happened as the last time. So now I know it is not the resistor. What could it be? Fuel? Ignition? Both? I got ride home from a neighbor and went back about 3 hours later. Jump start from the Cummins later and she was running like nothing happened. So now I think it is temperature problem. I was also concerned that my battery was not charged after 25 minutes of driving. It has a new red top optima in it and a new alternator as well. Maybe the optima take longer to charge I guess. Well thats the story sorry it is so long but I thought the more info on what happened the better. Oh it has a 318 and a 904 trans. thanks again.