Based on the description of symptoms, I'd also suspect a fuel pressure issue. But working most of the time, and having fairly similar circumstances each failure, suggests it's not a regular fuel pump failure from age. I would still suspect something electric.
There are some remote fuel pressure gauges available that could help verify that you are losing fuel pressure when this happens. But because of the age of the van, I'm inclined to recommend just swapping the pump anyways. It's eventually going to need it, probably sooner rather than later. But also, a likely place for an intermittent electrical connection to the fuel pump is in the fuel pump assembly itself. The wires in the assembly are small, and some broken strands, or a loose connector, could be affected by fuel sloshing.
The inertia switch is a good suggestion as well. It certainly would not hurt to push down on the reset button and make sure the "ball" is seated all the way. If you look at this cutaway pic, you can see how a "loose" ball can can an issue. Normally these are not very sensitive, but if the dirt gets in them, it can make the ball ride high. Ford did have a issue with 90's Rangers having faulty switches. I suspect the E-series switch is the same part, but maybe mounted in a more protected location. Also, make sure it's sitting vertical. It it got mis-aligned from other work (stereo or power mirror, power windows & locks installs are both likely to need to move it temporarily, or a vacuum line repair) and possibly not put back how it should be.
Vanna
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