sputtering/back firing

recently my peice started having a some problems, backfires under minor load, but not WOT. and sputters as well.

recently the plugs were changed, the wires loook good. the cap and rotor was also changed lately. also just changed the o2 sensors.

could it be a fuel problemo? sound more ignition?. the car starts right up every fire no problems i believe the back fire may just be from a small header leak

3 weeks ago or so new head gaskets were installed and an f cam. car ran fine till as of last week
 
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