Did you ever get your engine running correctly? I have a 115s that is behaving similarly. After a short period of bench running for break-in, my first 2 flights were good. Since then, it's got an intermittent midrange stumble on the ground. When it seems to have cleared itself up, I take it up and the stumble comes back within a few passes resulting in a deadstick landing ... usually off-runway.
Nothing obviously wrong with engine ... OS F plug, Wildcat 20/20, Xoar 16x8. I don't know my low-end air bleed setting off the top of my head, but I did set it per instructions. It will idle around 2000 - 2100rpm for a few minutes, no problem. My high-end needle is set rich as the engine is still technically in the break-in period at 2 turns out. It performs well at full throttle with a good smoke trail.