I hate do diag. without the car in-front of me but...
You're warm weather area. Twist the plug wires, with the engine off, to release them from the plugs. Gently until you feel them turn. Then pull them free. Look for ghost tracks up the side of the part that runs into the spark-plug tube. That would be side-arcing.
Second, pull the plugs and look for black carbon trails up the upper, porcelain, section of the plug. Will look like someone took a No. 2 light-saber to it. That is side tracking.
If either. Replace both plugs and wires with good, not what the counter person says, parts! If neither, pm me. I own Subies and an MX NA. Also worked on cars, for a living, since I was 16. 46 now.