Try increasing engine speed slowly in neutral, and if it starts to miss at the same RPM as 65 mph it may be too much vacuum advance. I had that issue with a slight surge on the highway with mine.
I did the timing by vacuum and it’s normally just right at 20 psi. I actually don’t have a tach in the car yet, definitely on my list but just haven’t done it yet, primarily because of all the electrical still to be done. I won’t be able to check the RPM that way is what I mean
