This is text book bad plenum gasket(or leak). Long ago when i first got my ram it was doing the exact same thing. All new parts(plugs wires cap rotor cam/crank sensor) Even checked the coil with a timing light under load to see what's happening. Then you will see why it backfires threw the intake. The vac leak in the intake throws off the pcm and it WILL fire when it's not supposed to(prior to tdc when the intake valve is open). After reading other posts on the subject i replaced the plenum gasket with the Hughes kit and problems solved. Never had the issue return in 15+ years. There also was poll when most people replaced their gasket(miles), You are right in the middle of when most go bad. Not saying it's 100% your problem but if i where to put money on it i think it would be a good bet.

 

Thank-you Moparite, I will investigate and this is a van and don't look forward to such a job. Does that mean the bottom plate gasket under plenum ? And do you recommend the aluminum plate as a permanent fix. Hugh's Kit , where can I find it?

 

There are a ton of threads on the plenum issues in the second gen ram truck section. Don't visit here to often but the problem is for any of the V6-8 magnum motors no matter what they are in. Yes i recommend the hughes kit with fel pro gasket's. The factory only used a thin piece of sheet metal as the plenum plate. It bends and allows the gasket to leak. You can see a pic in the link below.
http://www.hughesengines.com/Index/p...p?partid=27091

 

To all who replied , FYI, Leary of weak spark and wanted to fix it first. Replaced ignition coil and problem of backfire solved. thanks again group , good to know help is available. Delta.