You might just be too rich on the high end. What nitro fuel are you using and what plug? A couple of times now I've helped out someone with a new ST engine and they've all acted a bit odd when rich, not the typical smooth blubbering 4-cycle a lot of engines do, instead they surge and are hard to keep running. I'm not totally sure, but I think it's because the ST seem to run a bit higher compression. I suspect (but haven't tested) that the surging might also go away with lower nitro fuel, like 5%. I can't say for sure that this is the problem, but it might be worth a try.

Btw, you did watch the fuel lines while running at full throttle, right? No air bubbles visible going in to the engine?