Your probably not actually having detonation but I would pull the head and look if I were you. Your probably having lean plug burnout. It would appear my friend that you are at the other extreme. at that altitude the air is so thin that you could be suffering from to low compression. whats happening is you are having to overlean the engine because of the thin air to make power, because the Dynamic compression is way down. In your case I would go down to .014 head clearence and go to a hot plug and see if that does not help. I would use 40% fuel if I was in that condition.

There is a track in Mexico that when people go there you typically need a completly different head button to correct this condition. I cant remember if thats pegaso or not. I personally have not been there but I remember Ron Paris talking about it.