High voltage leak, a breakdown in the insulation on the spark plug wire or the insulation on the plug itself. It could run okay until you increase the throttle. The additional air/fuel mix in the cylinders raises the resistance to the spark plug gap. If there is any breakdown in the insulation anywhere, it could allow conduction to a ground instead of thru the plug.
Thanks Dave for the above info. I will pass it on to Bryan tonight on the SBX.