I would put another OS8 in it and run it. It sounds like the same plug is still in the engine that was used to break it in, so I am betting the element is contaminated with minute particles of aluminum and nickel from the piston and sleeve seating. This is quite common, and I always run in my engines on the bench for 3-4 tanks and throw away the plug before I put the engine in a plane.

I am also not a big fan of Pitts mufflers. As I have posted before, I have never seen an engine run really well with one. Some engines can run pretty good, others poorly. Some plug one outlet for more pressure so the engine can idle, but what does that do to the top end power?? I would try a stock style muffler if you still have tuning issues after you replace the plug. If you need an in cowl type muffler that actually makes power, call Dub Jett. He makes a really nice one, but it is kind of spendy. Good luck.

Scott