It would be almost impossible for that style of cam drive system to jump a gear tooth and it if did' the engine probably wouldn't run. Maybe a sheared off woodruff key that lets the cam or crank gear slip? I've never seen that happen. One way to check would involve watching the valve action in relation to the pistons- - - -#6 valves "rocking" as either #1 or #6 piston is at the top of the stroke. It would require a bright light or a borescope through a spark plug hole to see the piston movement.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!