I'm guessing the motor & or the geabox is the problem. You stated after several fantastic flights, and that you have good control surface control on the ground everything is doing what it is supposed to yet at full throttle it it won't stay in the air. Brushed motors & cheap plastic geared gear boxes both have a very short lifespan. Again you stated that all control services are functioning properly in the air too, you just don't have enough consistent power to keep it from stalling any more. I'd be willing to bet that the brushed motor is heading south or the gears in the gearbox are stripping out or both.