Well another common problem after installing the M-1 is the backfire through the TB. You can go to the dealer and have them check your fuel sync. Should be around 0 or +1. Another way to help is to run a dedicated line to the map sensor off the M-1. You will need to pull the elbow off the underside of your TB and put a cap on the barb going into the TB. Then run a line directly to the map sensor itself off the M-1.
"i would say your timing is off...maybe you put the distributor in wrong when you installed the m1."
The distributor doesn't have to be removed to install the M-1. So I don't know if it was taken out for another reason. But timing is pretty easy to check too. So might be worth a shot for peace of mind.
Well, as much as I hate pulling up an old thread, my 99 Durango Slt 5.9 is suffering from this same problem. Mine is all stock. It started backfiring a couple months ago, and has slowly gotten worse. I replaced the dist cap and rotor and have replaced the plenum gasket. It got new 3923 plugs about 8000 miles ago. My next step is going to be a leak down test and new plugs and wires. Just wondering if you found a resolution for your problem?