Put a timing light on the damper while you rev it up till it starts missing/running ruff. I bet you will see spark scatter. Then disco the vacuum and recheck, probably smooth, but if not, you have at least determined its either mechanical or vacuum advance

I went through the same thing and cured it with a new MP distributor.

BTW, I just put the same Summit fan (18" version) for the same reason.
GOOD flex fans rock!