It is likely leaning out from getting hot. its got too much load on it. You need to be at least 500rpm higher. Put a smaller prop on it or stick some nitro in it.

12-9 or 13-6 APC on 30% heli fuel is about right for the RPMS. you need to be between 10,500 and 11,000 So if you are using this prop already and less nitro then go to less prop. ANd if you are using 30% heli fuel and more prop put a smaller prop on it.

There are lots of combos but you need to hit the target rpm for the engine.

Another option could be poor cooling. And yet another problem could be a damaged engine from being run too hot or with too much load. Then no matter what you do you can't get it to act normal as the ring could be scorched and leaking past...all of these are signs like you describe when the engine gets up to operating temp.

Troy Newman
Team YS Performance