Andrew, Could be the dizzy cap or wires. If it's cracked or has a carbon path across it at that position. Easy to swap dizzy caps to try it. Or just look under the hood when it's dark and with the engine running. Look to see if there are any leakage paths around the cap or wires. You may see arcing around the cap or wires. Could be a bad connection on wire #1 at the plug or at the dizzy cap.
Tom
Tom