Axial travel of a distributor shaft will cause a "jumping" signal as the helical drive gear twists the shaft ever so slightly as it moves up and down. Shim kits are around that will take up the endplay a bit if you find that is what is happening. Requires pulling the distributor out of the block and checking the clearance of the stop ring pinned under the main body.
A slipped or slipping outer ring on a damper will move the TDC mark slightly from the initial position - but I haven't seen one that moved around while you were watching.
The valve train might have a cylinder with differing lash - have you gone through the drill of setting all of the valve lash?
Steve Metzger Tucson, Arizona