When I have experienced Vault hub covers falling off multiple times even after being installed correctly using red 271 (or similar) thread lock, it was usually caused by a crack in the rotor hub.  When the rotor hub gets hot the crack will allow it to expand enough for the Vault cover to fall out.  The crack may be so small that it isn't even really visible.  Replacing the rotor hub has been the most consistently effective solution. 

From my experience, most intermittent or random electrical issues are caused by loose, worn, or corroded connections.  Most PPS failures I've seen tend to occur as the throttle lever is initially moved from the neutral position to the in-gear idle position or at the wide open throttle position. 

If you do remove the intake plenum cover to access the HVS cap, rotor, and ignition wires, I would use that opportunity to swap the #1 fuel injector with the #7 fuel injector to see if the misfire fault code changes.  The HVS cam angle should be checked with Diacom to verify it is at 700 to 710 degrees when the engine is running above 1200 RPM to prevent ignition cross-fire.  Also, the fuel rail pressure should be 58 to 62 PSI.  Anything below 55 PSI or above 64 PSI is a problem.  Make sure the flame arrestor is clean and in good condition. 

Any excessive vibration in the powertrain or running gear could be interpreted as a misfire by the ECM.  Damaged propeller or propshaft, poor engine alignment, loose raw water pump bracket, or even a damaged flywheel drive plate could cause a misfire alarm.

IIRC don't the 8.1s have a "taller" trans ratio (like 1.2:1 instead of 1.5:1)?  In that case I'd go with an agro 15" prop like the acme 2419.

2 hours ago, GaleK said:

I bought from place @Bozboat mentioned above, then found them cheaper at Bakes!

Oh that's terrible, how much money did I cost you?