OMG need help fast

Change the #2 spark plug, then report back!! Reason is, we had a 340 Challenger years ago, it was popping through the exhaust. Changed everything, timing chain, carb, ignition, everything... that is except the spark plugs, which were "new". Well, one had a cracked insulator, never saw it and the plug looked fine. Several $$$ later and more time than it should've took, one plug and problem solved!!! At least it's isolated to one cylinder, it should be easy to find!!! Good luck with the diagnosis, Geof