I have a ST 75 with a tuned pipe that eventually developed possibly similar problems. Originally I was running 15% in it and it would idle great but would surge at anything above 1/2 throttle. I changed to 5% and the problem went away. The other day I had my Ultra Stick 60 out (with the piped 75) and forgot my 5%. Put 15% in it again and the surging returned. Would run well enough to get it in the air and then start surging and finally deadsticking and HOT!

Have you tried MUCH lower nitro?