NGK R5671A-9. I can get about 5 runs on a new set before the motor starts to miss above 4500. 408 blown application with water injection. If I clean them they are good again. Thinking about going to -8's or 7's and or running a little less water. Didn't have this problem when I was running cast iron heads. Seemed to crop up when I switched to aluminum heads? Thoughts??