This sounds like a simple case of too much elevator travel, which stalls the wing. When setting up a new model, I take it safely high, and apply full up elevator at full throttle. If the model completes less than one full loop before going badly off line and yawing all over the place, I reduce the elevator travel. If it continues to loop, while holding its line pretty well, I increase the travel, until I get close to a full loop before the model goes crazy. I have seen too many fine models written off by excess elevator travel, which can result in a snap roll into the ground when the flyer applies full up elevator in a dive pullout. This does not apply, of course, to models that are set up for so-called 3D flying, that needs excessive elevator travel, but it is best, I think, to use high rates for 3D, and set up the low rate elevator throw for a non-stalling turn.