The problem isn't with your GTB at all. Is this a very early version Spektrum system? They had a lot of problems with the early systems doing exactly what you're talking about. What happens is that your motor is drawing so much amperage that the RX goes into fail safe mode. There's not enough power to keep it on. I think you can send it in for a reprogram. Check the Spektrum site to be sure or give them a call. You can also try changing the RX for a new one. But most likely, you're going to need to get the cap that Spektrum offers. It'll compensate for the momentary loss of power when your motor is pulling major amperage like on take off or when you're hard on the power coming out of corners.
Tony