Stalling while driving

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I own a 1992 Ford Explorer. About a month ago, it started to stall out. It can stall out while I am driving it, or when I am at a stop. Sometimes it will not crank back over. Other times, it will start right up again but once I put it in gear, it stalls again. You can let it sit for about thirty minutes and then start it right back up again and it works fine! There are times you can drive it for days and it never stalls. We took it to a garage and they thought it was the crank shaft sensor which was replaced. I drove it that night and the next day all over when it stalled again. it is now back in the garage and we have no clue what is going on. Can someone HELP!?