you need a cooler plug with higher nitro. higher nitro means higher pressure and temperature, and more pre-ignition, so a cooler plug is in order. I've had a couple engines that would run for a couple seconds and then die on 30% nitro but were fine on everything else. turned out there was so much pre-ignition it was basically just killing itself. put a cooler plug in and added a head shim, ran great after that.

anyway, overall idea is correct though, plug is probably too hot. shim your head or go to a cooler plug. (just adding a second washer under the plug can sometimes make a big difference also!)