Starts going off when a higher speed is reached

THis is a weird one! The only thing that makes sense is some sort of interference that is impacting the electronics inside the detector. Sometimes ignition "noise" can cause this behavior. I have seen ignition noise so severe that a Garmin GPS would not work inside the car.

Can you test this without moving the car? If you rev the engine while the car is parked, does the detector still go off? This will allow you to determine if there is some form of intermittent operation inside the detector caused by solder joints, etc., or a problem associated with electrical noise.

BTW, what type of car do you have?