Usually a high speed miss is lean condition or ignition wires causing a miss or too much ignition advance or some other combined problems, the gray smoke is probably "oil smoke" & either going by the valve seals/guides or the rings have blow-by, not completely sealing the cylinders, does it have excessive blow by out the valve covers, have you done a leak down test, to see if you have any leakage into the intake, into exhaust either indicating valve seal on the seats problems, too loose or too tight preload or valve lash, or by the piston rings into the pan... We need more information really, just to give any educated guess, it could be a lean fuel condition, also have you checked valve lash/preload clearances/valve adjustment, it could be a broken or weak inner valve spring, it could be cap &/or rotor also, are there any orange dust residue in the cap ??, could be an ignition coil problem possibly, plug &/or wire misfire problem too... what kind of ignition & induction/manifold system does it have & what type of fuel system, pump & carburetor/s, what jets ?? etc... IMHFO just having a light gray plug doesn't really tell enough of the story, to diagnose much... how light compared to the other plugs ??... have you even checked any other plugs ??... to compare them, then how far is the heat going down into the plug ??... what heat range of plugs are you using ??... what is you total ignition timing at speed ??... are you running a vacuum advance ??... have you done any mods to the distributor, to change the ignition curve, like spring &/or weights ??...
