You've tuned and flown other O.S. Max engines, and you say you've done your best to dial in the low speed needle. Let's assume it's not the low speed needle setting itself.

If you're not using an A3 plug now, try a hotter glow plug like the O.S. Max A3 or the F-type 4-stroke plug.

If you are using an A3 plug now, try an idle bar glow plug like the Fox RC Long plug.

If a change in glow plugs doesn't improve things, you might try a different fuel mixture with slightly lower oil content. You could add a little bit of straight methanol to your existing fuel, or try a different commercial blend with slightly lower oil content. Too much oil content can cause the transition problem you're describing, too.