You gotta run a cooler Plug..2 levels cooler then all motor plug,,you also need to close up the gap from 25/26...i have see Big inch NOS motors running 22 gaps.
also have you manually checked your timing retard to make sure its pulling the timing out when you pull the trigger? (critical)
Next how are you feeding the fuel to the motor and Nos ?..did you wet flow the Nos to make sure your fuel pressure is holding upon activation ?
No doubt you got some detonating going on there.
Brian
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Bjuice..
"I'M YOUR HUCKLEBERRY ! "