Cost index set to 80
VNAV page shows, .80 mach

Upon Leveling off at FL340 speed holds for about a minute, A/T commands .84 to overspeed.

Speed set in the FMC VNAV page to 0.77 M and system holding .8 to 0.81.  MCP setings still have to be set to about 0.04 mach below what you actually want the plane to be at.
P3D v4.5+HF
Active Sky wx
Environment force
Swift client

I understand that in real life the speed is not held perfectly but to overspeed at a cost index of 80 seems off to say the least.

Speed seems to hold better on descent, but not without constant speed brake deployment.

What I would like to try is a complete (by complete I mean COMPLETE) uninstall of 1.4 and start over.  What registry entries need to be blown out to do this? What folders outside of P3D Simobjects and Captain Sim folders should I delete?