If you have already checked every thing out,just put in a new plug and see how it runs. One last thing,make sure head bolts are tight. I've pulled that one before. I have found with go-tecs if you take apart,clean,seal, you really never have anything go wrong with them. At least that's been my experience. I always would think whats wrong with the engine when in fact it was always some thing else. Here's my last goof. Had fuel line coming from filter a bit to long. rubbed a very tiny hole in the line.Engine running funky. I checked every thing. Finally just got pissed replaced all the fuel lines found problem. I had even ran my had over it. But from what your describing sounds like fuel restriction of some sort. Good luck!!