Bought a sled a year ago. Ran rough. Everything checked out. Good fuel. Fuel pressure. Had spark. Barely hit 6k with belt off... fought and fought. Finally lost spark on a cylinder thru some old used resistor plugs in it. And fired up. Pulled high rpms. Belt back on and ran great. Talked to a guy at the shop and said he has had that happen. With such a small electrode they damage easily

 

maybe you have a hairline crack in the porcelain by the electrode you can't see. i've had this problem on many types of spark plugs. even new out of the box. replace one with an original to see if the miss goes away.