John,

If the van runs OK for a while once restarted immediately after acting up
(i.e., with no time to cool), then your ignition system is most likely all right.
 I had a van exhibit symptoms similar to what you describe which were caused
by a faulty TEMP II sensor, but it didn't take 20 miles of warm-up to appear.

Nonetheless, you can pull the TEMP II sensor and check it by heating it in a
pot of water while you plot its resistance versus temperature characteristics.
 Or, whenever it is acting queer again, substitute a resistor of a value
appropriate for the engine temperature at that time.  You can pick the correct
value from the resistance/temp chart shown in the Bentley.  I am not certain if I
remember correctly, but the value of 200 ohms for a fully warmed engine comes
to mind; better check it for yourself.  If the engine smoothes out, you need
to replace the TEMP II sensor.  The connection at the TEMP II sensor queefs
out especially when exposed to the balmy weather in sunny Maine, so make certain
it is sound before you start.

George