At high field elevations, it would be difficult to get the carburetor balanced between high and low RPM mixture settings. The air's too thin to get a good 'set' on the low end once the needle's correctly-set for top-end. The idle is too rich...to get a good idle, you have to lean it so much that your midrange goes out the window.

You may wish to try a hotter fuel. The added nitro is an oxidizer, so it adds power by allowing more fuel to be burned. This may help your transition problem. The only other solution is to accept a higher idle RPM and prop the engine so that you can slow the model down enough to land.