Steven- as you've already found out, it's difficult (and expensive) to troubleshoot unless an OBDII code locks in, or the van wont restart at all. It sounds like you are on the right track with the hardware & software you purchased- you've got the deluxe setup at your disposal
With the symptoms you describe, I would suspect a crank or cam position sensor issue (I'm not sure which the 1997 3.8L has). When these go out, you loose signal to your ignition coil(s) and loose all spark . O2 sensor(s), EGR, or mass air flow sensor issues USUALLY do not result in a complete stall & no restart condition. Do you get a gasoline smell while attempting a restart when this happens? (indicative of a no spark, but plenty of gas condition). You may be on the right track with something (sensor) getting hot after an hour at hywy speeds, causing it to malfunction.
G/luck
Joel