Ok, does it show any RPMS when just sitting there off? So U now know that for that short while it is "Off"!!! Something in the "Ignition Module/Computer" or "Support Wiring" for it, is causing it to drop out. The "Backfire" is due to the gas flow continuing while the "Ignition" drops out. When it comes back on the "Spark" "Explodes" that gas. The "White Smoke" is from the worn "Valve Guides" passing oil down the stems & into the cylinders. Check all the wiring for lose connections, bare spots & if dealer can test Ur modules, take them in. There may even be a "Recall Notice" on Ur "Module"??? John