The original needle valve assembly was a cast-on part like on the .25/.46/.50FX. Apparently, heat, especially on the .91, ran up the aluminum stem and vaporized the fuel in the needle valve area, causing the bubbles and erratic running after 5 minutes of flight. The later ones have the same assemble as on the OS .65LA and the .46AX. This has a brass insert with a black plastic housing and the same needle. A sheet metal bracket attaches it to the rear cover. This one works well.

It used to be that Hobby Services would give you one of the new needle valve assemblies. Probably if you bought it used off the web, it has been run so lean that the cylinder or ring may be warped and it will never run well without some parts replacement.

I had one of the original .91FXs and had the problem. I bought a K&B .61 remote needle for $15.95 and the engine ran great. Try an Ultra Thrust or Jett tuned muffler for a thousand more rpm.