You could be just trying to idle too slowly. Inverted engines don't idle as well as upright or side mounted ones, no matter what kind of plug you have in them. It's also common to see onboard glow ignition hooked to the throttle channel on inverted engines for just the reasons you are discovering. There's a really cool (a bit pricy though) gadet out there that provides current to the plug based on the plug's condition rather than the throttle setting, I've heard some really good things about it, but I keep forgetting what it's called or who makes it. (useless info, I know).

5_spot, actually, I personally use K&B plugs because they are a bit longer (and a lot less expensive) than the OS plugs. I get a few extra RPM out of the engine that way with no problems. Sometimes a little extra compression is a good thing .