Only 2 things can make an engine come back to idle, one is the servo closed the venturi or the venturi closed by itself. Bax is right about getting the engine out of the plane and on a test stand...it is very possible the engine is over heating. There may be a small piece of crap in the spray tube...very hard to say but getting the engine on a test stand or remove the cowl and run it on the ground for 5 minutes to see what happens. I have owned several ST .91's and don't recall having to many problems...I find the Perry carb suggestion interesting if a 700RPM gain is what I can look for. You will figure it out soon no doubt...let us know what you find. Good Luck.