My YZ250F loses all its power after it warms up.

proabably thermostat if you changed thermostat and still have problem it could be defective ive heard of a dodge pickup having same problem loses power when heated up it was plugged catalatic converter.

If you have a vent hose coming out of your gas cap make sure it is not kinked. A kink in the hose will cause a vacuum in the gas tank and very slow gas delivery to engine, which would become apparent at higher engine rpms including after warm-up.

Might be a modual needs replacement

If you drive a lot in town, it could be your catalytic converter is getting plugged up.

HAVE THE CATALYTIC CONVERTER CHECKED FOR BEING PLUGGED.

check the intake gaskets. in the middle front and middle rear. If they are oily, it more than likely sucked part of the gasket in. When it warms up the leak gets bigger. The easy way to check is to get some carb cleaner and spray all around the intake while idling and warm. DO NOT SPRAY ANY EXHAUST PARTS , carb cleaner is combustible. The truck should stumble or even shut off when spraying with the carb cleaner if there is a leak.

sounds like an ECU fault.one of the chips is breaking down when the units warms up

Check fuel feed is ok and fuel pump pressure also check cold start injector could be running on this til warm.