I have checked the obvious things so far - distributor cap, rotor, spark plugs and wires were changed recently and look OK, not touching ground or anything hot on the engine.
Vacuum lines are also ok, not loose, cracked or disconnected.
The only thing we can rule out so far is the fuel system - this van has dual gas and LPG fuel systems, problem is the same on both.
I wonder what we could try next beside pulling plugs to check for spark/fouling.