IF my memory serves me correct 71n/59f jetting is around 200 hp , what is the total timing on motor? , 34 degrees total spark lead @ 10.1 cr seems a hell of a lot of advance when on the bottle , maybe post a pic of those plugs , did you drive back to the pits with those plugs? , heat range? what fuel pressure/bottle pressure are you running? , without this info it's a crap shoot , i'd say the motor slowed down due to detonation.

About the only other answer you'll get here will be give Monte Smith a call.