I had a similar problem with my old van and it turned out there was 5 gallons of water in the fuel tank and we needed a new fuel pump. After all that was fixed we also had to have a new distributor cap but I think the water in the tank was causing the most problems.

The problem was so random, it would start and drive just fine and then just leave us stranded. After several tries to get it started it would sputter and stall unless we could get it opened up on the highway.