I've had this before, and the key word from the OP, is at speed. It NEVER overheats while stood.
So therefore everything works, no thermostat blockage, no pipes blocked, no airlocks.
What happens is that the air pressure going into the car is greater than the electric fan motor can provide, thus the fan stalls, and car overheats, as the brushes are fubar. They work OK at low speed, but under pressure, they fail, and stall. Been there done that, got the book.
Mine was manual, by a belt, and the fitting was the wrong way round, I even fitted an additional fan to compensate.