Truck loses power while driving, surges and refuses to restart for a while.

1991
Mitsubishi
Mighty Max
(Not sure engine size)

Before I bought this truck, the previous owner (friend of mine) put it into the shop because he thought the alternator was going out. So the alternator was replaced a week prior. (I am checking to see if it was a new alt or a refurbished one, just waiting on response)

I drove the truck the first day and had no problems. That night I drove it to a taco bell, then a gas station (Distance of about 4 miles) while it was dark. I had the lights on, the radio on, and my dash back light lit. When I got to the gas station, I ran in and out in literally less than two minutes. The truck would not start. I checked, and I did not leave anything on. We had to leave the truck and come back the next morning.

The next morning, I replaced the battery with a new battery, and it started fine. However, while I was driving it back home I noticed that the radio kept turning off. Also, while stopped I noticed that the idle would "surge" or fluctuate noticeably. I stopped by a friends to have it looked at, at which point we turned it off and on a few times and it started and ran fine. We even turned on everything (Radio, lights, A/C, etc) to see if it would die. It ran fine, so I figured the new battery was working and that was the problem.

That night I drove it to a friends house and when I left I got stopped at some train tracks, and after about 20 seconds of idling in neutral the truck died on me and would not restart. We pushed it off the road and opened the hood. When we tried to jump it nothing happened and I noticed a spark plug was undone, so we reconnected it and it jumped fine and drove fine back to the house. I had to stop by and drop the friend off, but when I restarted the truck it started fine. As I was driving, about a mile down the road, I stopped at a light. When I tried to go, the engine started losing power and spluttering. I noticed the radio shut off and the dash clock was so dim I could barely see it. I gave it a lot of gas and it spluttered for a block or two. Each turn (3 turns) I made to get back home the truck surged or spluttered and didn't want to go. When I parked it I jumped out to see if that spark plug had come undone again but it was still good. I tried to restart the truck and it didn't start at all after that.

The next morning it started once a few times as I was checking things out, then stopped starting. Today, it started one time. I turned the radio on and the a/c on, but when I turned the headlights on the radio turned off. I turned the truck off to see if it would restart, which it did not.

The truck has never had any problems and even drove fine for a few days after the alternator got replaced/fixed. I have a full tank of gas in it. New battery. Any help would greatly be appreciated. Also, if I was not thorough enough, just let me know what information I need to supply in order to give a clearer picture.