A little throttle management may be in order here. Please remember that electtrics require much more throttle management than do glow engines. The throttle is not an on-off switch.

When I flew my electics for the first time, I found that going to full throttle for take off then learning how to fly them with the correct throttle response, meaning, finding the throttle setting that gives you a good straight and level flight and then leaving it there unless doing maneuvers, is hard to learn, although I fly my glows that way. The batteries are not like fuel tanks on glows. When they begin to run down, the more that you demand, the less you have. Then they begin to surge like the OP described.

CGr.