I don't have a modern norton, but occasionally someone gets this on the old version where it just shuts off, then magically restarts like nothing is wrong, and it can be a worn out ignition switch. The other thing that comes to mind is a skinned wire somewhere grounding out against the frame, or in the case of the old bikes, somewhere inside the headlight shell...
Other than that, I would always change the spark plugs first, because if the insulator cracks up inside the plug, it may test fire when held against the head, but inside the cylinder that spark may arc up inside the plug's body at the insulator crack occasionally where it won't ignite the fuel...