1999 Ford F-150 clueless

I have a Ford 150 Lariat. I hadn't used it in a bit do to ownership problems. That cleared so I drove it here and there and went to wash it all in the same day. Everything was fine till I wanted to leave the carwash. I started the truck then it died after a couple of seconds. Restarted and it worked. Hmm. A block later at a stop light it died again. Restarted it and it died again. I noticed that everytime the rpm dropped below 1 the engine rumbled then died. When gas was applied it stayed on. When gas was released below 1 rpm it would die. I got home by keeping rpm above 1 and used the break when needed. Got home put it into park and the engine didn't die. Hmmm. Any solutions? Thanks.