Van "slips" into neutral randomly

What is your question?

We have a 15-passenger van that has started slipping into neutral randomly at speeds above 40MPH. Has happened three times so far. Without warning, the van will studder for a moment, the speedometer will drop to 0, and pressing the gas will result in higher engine RPM's but no increase in speed. The first time it happened was during a heavy rainstorm and we thought maybe it was from driving through puddles of water on the road. Tonight it happened again, twice, at highway speeds with clear weather conditions. Pulling over, shifting to Park, then shifting back to Drive seems to "reset" it.