Overheating problem

My van is overheating, when speed for like 2 minutes, it will drop but when you off the engine for like 2 minutes, it will speed up again, the van is misbehaving, I have replaced the fuel pump, fuel gauge, changed nozzles, wash the radiator, check the coil and plugs. Please what do I need to do next?
Both fans are working, It doesn't stop at all. This is how it happens. When you drive it like 2 minutes, the engine gets heated up and when you put the gear on neutral and off the engine on a sloping area, it will accelerate well but the moment you drive it like two minutes. It will not speed well and the radiator and engine will extremely be hot, then the water level will go low.