[Tinker]

As Flatjack mentioned, a compression check should check it. Don't make it harder then it might be. Granted sticky valves can show up under load and speed also. But at speed I'm not sure your pulling sparkplug wires. Then sticky valves underload... maybe mmo it a few times. (could use some diesel fuel bottle fed, but I prefer mmo over time or for results. Don't fill the tank with diesel!)


Vacuum gauge would show a dip or flutter. Compression gauge might lead to the cylinder it's happening.


Not saying you can't with a open hood and someone sitting on the fender. My pops and uncle drove a car with the uncle running the throttle on the fender and my dad driving for a few miles to get it home. Different time. Compression checks are easy and should be done on any engine.


If after you check vacuum and compression (standard mechanical diagnostic tools for old engines). It could be a miss fire from the electric ignition. I'd check the earlier first. Easy tools and checks.