I only get a PO300 when I'm on the interstate, pulling my travel trailer and coasting downhill...and not on every trip. Go figure. My solution is to not worry about it right now. I bought plugs and wires from Rockauto and will replace soon. I never get a code when not towing. Since I don't tow but a few times a year I just clear the code and keep on truckin.

If a wire is grounding it will totally cause a misfire. If you saw "dents" in the wire it's highly likely it was grounding. Good find!