Timing curve mystery

I would "rig" a way to test the problem by elimination. Best way is distributor machine that can operate off breakerless distributors, but there are other "ways."
1...Spose it could be a problem in the cam/ distributor drive
2...If you can, rig something like an electric drill to drive the distributor(s) for testing. Remember that the distributor rotates at 1/2 speed, so if you are testing it up through say, 2K-2500 engine RPM, you only need half that to drive the dist.
3...Maybe consider buying a 4 pin GM module and wiring it up with a used coil and plug so you can trigger your light. You will need a rigged degree wheel, even if home made, to document results
4...Substituting a non "dial up" timing light is a good idea