a condensor when it goes bad causes some nasty arcing across the points and you can see this through the apertures of the spinning flywheel.might hook up a remote fuel tank and run the engine and rev it up while looking to see if the coil wire or coil itself is breaking down,you may see sparks which means the coil is shorting.also look for cracks and or breaks in the spark plug high tension lead or the case on the coil itself.
may also have a wire in the harness that has a intermittent short and the frequency of the vibration increases with revs and that could make it more apt to short.
wish i was there to look at it with you as this one has me very curious!
