What is the resistance of the plug wires? What type of wires are they? Try swapping the plug to a different cylinder and see if the problem follows the plug. Then try the same thing with the plug wire. How are the plug wires routed, and are they fitted in some sort of loom or bracketry that might interfere.
Are the plug wires fat yellow wires? I always hated those things. After all the advice and tips here, the only other thing I can think of is checking the air in the tires.
