Anyone have this happen? Where the mark slips or turns and you don't get correct timing?

Yup. Older motors. I remember in the 80's when a bunch of small block one's actually seperated. Boy where those noisy !

I saw one come through the hood of a 66 Nova-caine at about 8,000 rev's

Yup, I replaced at least 4 of them back in the 90's when I worked on cars for a living. If you let it go past the point of no longer having rubber in there, it will start creating a tons of tiny metal shavings. Then an unsuspecting mech comes along with his air ratchet pointed the wrong way and winds up with a bunch of shavings embedded in both eyes and a trip to the ER. Not a fun as you might think..

Yep time for a new harmonic balancer, the outer ring slipper in reference to the crank. After time the rubber holding the two together fails. As stated above a failure at 8000rpm is exciting to say the least and on a boat possibly deadly.

The stock balancers are not made that great. The rubber deteriorates, I can defiantly see them slipping.