Burning tips off plugs

Has anyone had an issue with burning the electrode off their plugs. I am running a 440 w/aluminum heads, about 440 Hp at the rear wheels. It's been on the dyno, and isn't lean.Autolite 3926
It only did this to a few plugs. They are not hitting anything either.I ran the plugs all year.Last time I pulled the engine down for rebuild, everything was OK, but the same thing, a few electrodes were burnt off.
Any idea's?
Thanks Guys