5.7 hemi idle and backfire

SSSSSSSSSSSSSSSSSSSSSssuper nice.

Since all plugs were sooted up, Ima thinking there is rich running issue that needs cleaning up. If you don't have an A/F on it,I would pop the plugs again after a short trip and see whats going on.
Sooted plugs do exactly what you describe, and are often irrecoverable.

It is also possible I suppose, that an underpowered ignition system could do that.. but I'm pretty sure I see COPs under that hood.