Intake Manifold Back Firing

My 1989 Chevy Astro van with the rebuild engine, developed this intake manifold back firing in the last few weeks.  Its not like exhaust pipe back firing, where it makes a loud bang, but rather it just makes a popping sound, muffled by the air cleaner.  The problem is it burns everything in the intake manifold, and you lose power for that second.  2 intake manifold back fires in a row and you can stall out in the middle of a U turn or what ever.  Hit the gas to pass someone, it pops a couple of times and your right back behind them.

This was ridiculously easy to fix, when I figured out what the problem was.  The cause can be a sticking, bent, or burnt intake valve, valve timing out, or ignition timing out.  Or I read bad fuel that is still burning through the exhaust stroke and into the next intake stroke as the intake valve opens.  Basically an intake valve is open when  the spark plug is sparking, or burning.

I thought the timing belt had stretched, but I've read they don't stretch like the old chains did.  So I thought I was going to have to pull the heads and do the valves.  One last ditch effort was to address the spark plug wires.  I went to the wrecker and purchased 4 spark plug wires for $4 of f a newer astro van.  Mostly I wanted the extra wire dividers, and extra loom that came on them , but I did replace the one wire that was arcing before.  I spaced all the spark plug wires evenly 1/4" apart, eliminated wire crosses,  put loom on them where anything touched them, and clipped them all up securely.  This fixed the problem of cross sparking, as seen in the video.  Hopefully this helps someone.