Electric scooter throttle help?

I have an off-brand stand-up electric scooter. It runs fine, but when you twist the throttle all the way, all power dies. It will slowly accelerate to about 60%, then cuts out. I can hold it at 60%, and that is as fast as it will go. I checked the wiring, and nothing is loose, or shorted. Should I replace the throttle, or is it a bigger issue (ECU?) I have some electrical experience, so I would much rather repair it myself, if possible.

Could be something as simple as the potentiometer or simliar device that is actuated by the hand grip (or hand grip cable) is bad. Potentiometers used as voltage dividers for control circuits often fail in a manner consistent with your description of effects, when a break occurs in their winding. The wiper controls things fairly normally, until it moves past the break, at which point the voltage source from the winding is suddenly gone, due to the break in the winding, and there is no control signal.