1988 Other Nissan Models Possible Elecrical

Okay, yes I am a woman, but I know a little about engines. Not much, but enough to know to ask. Truck was running good and all of a sudden the radio went off and the headlights went out & truck died when I stopped. The alternator was bad so I replaced that & put new battery in it. It ran good for a few days then one nite I shut off to go into a store, came out and it would not start. I had lights, and bells, but no crank. Popped clutch to get it home and check it out. Figured it was the starter. Popped clutch and drove to auto store for starter. Got back home again, popping clutch. Next day went to start and no crank so I went to change starter and had trouble reaching the bolts so didn't get to change it, but when I tried the truck, IT STARTED. Every time. Like nothing was wrong with it. Again ran good for a few days and then it started all over again. My radio went off, my headlights went off. Brought battery back to be tested and they are charging it cause they said it was only holding 11 volts. Could this be a short some where & if so, your best guess. OR maybe a voltage regulator.I am very confused. Please help