if it weren't for the fact the problem seems to have appeared out of nowhere i'd have no doubt that you are hearing the typical "ping" of preignition, especially since it's happening under high load..
but since the combustion chamber's volume is not packed with carbon, raising Compresion Ratio.. and since timing is on the mark, what could have changed that would cause it? A bad batch of gasoline? A wrong spark plug that extends into the combustion chamber (or has something wrong with it's tip causing a hot spot)?
Anyway, i'd try a little experiment.. Add some thickness to the head gaket and take it for a ride up that hill.. see if it makes a difference.