My plugs keep crapping out

[smoker]

My spark plugs only seem to last about 500 miles then they start misfiring when i use high boost at high rpm. I have tried cleaning the plugs but only a new set cures the missfires. The plugs are the normal coffee colour and look perfect and are ngk bpr7e's. I can't hear any sign of detonation. Any help apreciated it's getting expensive!