Richrd...you've just described the vast majority of 2C engines on the planet. Lots of engines are grossly over-carbureted to help allow them to run at insane rpms, (and produce the impractical HP ratings that the marketing folks like) but unfortunately the rest of the speed range suffers from really poor performance.

You might continue to fiddle with the needles to arrive at some happy compromise, or you could run a slightly smaller prop which can partially alleviate the problems you describe. Other things might be to try a few different fuels (just to be sure), or a carburetor with a smaller bore venturi. A .28 doesn't need a canyon-esque carb bore to run well...or run fast.

Just some thoughts...