As Mettler says, the one thing that will kill a plug is getting a tiny bit of aluminium on the element. This could be either from some swarf or from running in. For that reason you should always use the oldest plug you've got when running in. But in your case it might just be that the plug is too hot for the amount of nitro you're using so try one that's a little colder next time. You really need a tacho though because the best heat range plug is the one that gives the highest revs.