A QUICK QUESTION ABOUT MANIFOLD VACUUM

While the rpm is up , and the vacuum is dancing, watch the timing marks. If they're dancing to the same drummer, put an independent vacuum source on the VA and repeat. If the dancing continues, check for timing chain stretch.
But if the dancing stops, blame it on the VA. To prove it, disconnect the VA, then run it back up to 1700 and simultaneously advance the timing to 34*. If the dancing is back, it wasn't the VA.
So then, Ima thinking your combo may not like 34* @1700 rpm.
So then, still at 1700rpm, and running 34* mechanical timing; just reach in there and retard the timing until it stops dancing, keeping the rpm at 1700. Then read the timing at 1700.
But if nothing seems to work, check your rotor phasing and plug gaps.
I have seen the VA pull the plate over, and the mechanical advance align in weird ways that causes the rotor to be marginally positioned and the sparks go to the wrong tower. And I have seen coils misfire, in weird ways when the plug gaps combine with lean AFRs , and the coil just says, OOps no can do.
And a common mistake is to run the Numbers 5 and 7 wires too close to eachother, causing induction crossfires, and then the engine gets two fires on those two plugs but 90 degrees apart. Not good if one of them is on the intake stroke.
Good luck