Just saw your video - any issues with the distributor advance weights? No broken springs in the distributor? Is the vac advance diaphragm arm engaged in the advance plate? If the timing chain is known good I might try swapping in a different known good distributor, or go through yours to be sure it's 100%. And make sure your plug wires are good - not breaking down or crossed sending errant signals to your light. That timing is jumping around a bit. Some guys run a small collar on the end of the distributor shaft that stays in place with a set screw. It pushes down on the oil pump drive, keeping it from riding up the gear on the cam instead of driving the distributor and causing timing fluctuations. Seen it used on drag cars mostly. How's the oil pressure at idle? There were a bad batch of small block oil pumps on the market a few years back that would have like 60 psi at idle. The cam drives the pump and distributor and issues with the pump can cause timing issues.