Frank makes a great point about the fuel pump…
And, like all have said, your external plug grounding test may not be an accurate indication of good spark inside the combustion chamber.
You mentioned disconnecting the vacuum switch, did you actually disconnect the knife connectors on the external primary leads coming out from under the flywheel (and isolate/tape up the exposed connectors on the magneto end?
You might try using the inline neon testers, or perhaps a timing light on the suspect cylinder while the engine is running to monitor spark. How is it that you are sure the bottom cylinder is the one that is cutting out?
The only other thing I can think of is a small water leak into the cylinder that kills it at lower speeds…
.
