Welcome to FF.
You have done well, and checked or fixxed most of what I would say to look at. I would say to check the plug wires and make sure they are tight to the plugs and the coil. You can also pop the hood at night, use a spray mister and mist under the hood around all the electrical wires and plugs IN THE DARK. Watch for arcing of any kind and listen for a miss.
Also, pull the plug on the cylinder the code is for. Does it look any different than the other plugs? Let us know.
Speed is only a matter of money; how fast do you want to go?