The arcing you see at the coil when the plug wire is disconnected is normal - the juice wants to go somewhere when the ECU fires the coil and it's jumping from the coil to the coil frame and/or bracket..... the easiest/nearest path to ground.

Since you haven't replaced the coil lead, that would be the logical next step. Actually, it would have been far more logical to have replaced it along with the plug wires.

If that doesn't work, check the ignition timing. If it still is acting up, you'll need to go through the complete diagnostic routine as outlined in the FSM.

Frank