Here is an excerpt from a Clarence Lee column;

A few issues back a modeler wrote in about an O.S. 91FX that would not run at part throttle. I have one that had the same problem. I also noticed a large amount of bubbles in the fuel line from the remote needle valve to the carb.
The problem was found by removing the fuel line at the carb and connecting it to my six shooter fuel pump. By drawing fuel from the tank through the needle valve I was able to duplicate the problem. When I removed the needle valve and plugged the hole, the problem went away. I tried a new O-ring and a new needle valve. No help there. I solved the problem by having another identical O-ring groove machined in the original needle valve with a space of about .030" between the grooves. I wish I could say this was an original thought. It is not. I found the two O-ring seal idea on a Tower Hobbies .46.

This is part of an old thread from another board;

There is light at the end of the tunnel! There was a follow-up article in R/C Report magazine that just came out yesterday about the .91 FX problems. A friend of mine in our club scanned the page and emailed it to me. O.S. has released a new part for those engines that fixes the air leak around the needle valve - a 6.5 mm o-ring. It's a little wider and a little bigger than the std. 6.0 mm o-ring that comes with the engine and is causing the problems. The author of the article stated that he installed the new o-ring and the problems disappeared. I ordered two of them yesterday!

None of these may fix your problem, but may give you more stuff to chew on_Bob