Is there corrosion on the threads or the tip? If your burning coolant, you can sometimes see a greenish tint on the spark plug tip(not the electrode).
How is your radiator cap? If its not sealing,your system loses pressure and it will evaporate causing a mysterious vanishing act...

Does it smoke at all under WOT or heavy loads/high rpm?
A leakdown test may be necessary for headgasket diagnosis. Good luck!

P.S. I have to say I dont like your threads title. Dont shoot techs! were just trying to help!