Dan,
Changing the sparkplug with a new will likely shows noticeable difference on revs especially when the governor is not working properly. You mentioned that the plug that just died, the revs is a bit too high and the new plug revs the engine to the point it almost self destruct. Obvious reason is because the plug is new and has stronger spark and strong ignition. But still that doesn't address the main issue and only the difference between plugs.
Possible solution:
1. Clean the governor air venturi with wire and carby cleaner. This air venturi is located on the base of the mower in-line with the flywheel with nozzle connected to the carburetor diaphragm cap.
2. Change the diaphragm. The diaphragm might be getting hardened and stiffer or you might have the aftermarket diaphragm which is hard, thicker and stiff.
3. Change the long spring or return spring with a lower weigh or grams (weight is measured by grams per displacement) e.g. the clear color one.
4. Change the main jet with lower flow rate e.g. 2-rings.
5. Adjust poppet valve setting to low or more restricted when fully closed.
Cheers,
CM
