GR - I recently had a very similar situation with my (relatively new) OS 91 FX. Low speed & idle - fine. Upper to High speed - fine. Mid range - well, it would wander up and down - pretty unsettling.

Inspection showed that the fuel tank stopper was damp - not wet - but some fuel was leaking - but just barely. I found that the screw in the stopper was no longer tight. I went ahead and replaced all of the fuel lines (inner pickup, fuel to carb and pressure from muffler). All is good.

Hope your problem is as straight forward to resolve.

Oh, how easily does your engine start? Iask because mine required closing off the muffler tip (not completely - just partially) to generate enough pressure for it to start with a starter.