Losing spark after 20 minutes of run time, then engine shuts off

Do this power the ignition coil from the battery and see if it stays on-if so check the ignition wire back to the ignition switch and also the ignition switch itself

Did you check the ignition switch coil primary wire for an open circuit and also the ignition bypass at the starter relay to the coil

Also have the ignition control module checked-

Tag the wire @ the ignition switch and run it to the coil + if there is a bad section of wire you will by pass it. If the van runs well run a new wire or trace it looking fore a brake with green junk on it

A popular auto parts store can test your module for free. Test 5-8 times, to warm it up. Yes, even new ones can be bad.I know this 1st hand. Tore my jeep apart for 2 days, because I "assumed" the new one was good. Make sure you test the new one before you leave the store! (Might make 'em mad, but you are the one dealing with the problem. And the customer is always right!)
Might also crank him up and start systematically shaking wires. You might luck into the area with a problem when it dies
Below is the machine for testing modules--there's a drawer-ful of connectors that go along with it for other applications.I have laid out the ones for testing jeep cjs beside it in my pics (on the left "prestolite ign" ('78 and earlier)---on right "motorcraft ign."('70s and early '80s jeeps and fords)