How about removing the cowl for your test flights? This will allow you to see any fuel line bubbles while on the bench - between the tank and the high speed needle assembly and then to the carb. It will eliminate "overheating" as a source. Guessing that there's a remote fuel filler - might remove this variable while the cowl is off to simplify issues. After thought > removing cowl may be a problem if there is "nose weight" glued to the inside of the cowl.

How old is the OS 91 FX? There are several o-rings that might have been nicked during (re)assembly. The needle valve may not be seating accurately. It may be worn. The cowl may shift and put pressure on the needle when in flight. Just tossing out some rambling thoughts ...