bouncing timing mark

Mad Dart, I bought that same thing at the hardware store for my 360 w/ MSD dist. and my timing still jumps so don't get your hopes too high.
My motor does this same thing and I'm leaning towards a bad or twisted balancer, the rubber O ring thing gets old and dry and who knows what it's doing. Also if you have an MSD box it can cause timing lights to get "confused" with the 3 sparks at low rpm so it may look like your timing is jumping but really the light is flashing too early or late. So if you've got an old stock balancer on it I'd lean that way