Hesitation, loss of power on inclines.

My truck runs great while its cold. But once that engine temp rises and the engine is at normal temps, the engine light comes on. And once the light is thrown I almost instantly have less power (horribly noticeable on incline) and also it will hesitate and stumble during take off or acceleration (bad). It will sometimes gradually get worse the more miles I drive. Another thing that's odd. When ever I'm on an incline and I press down on the throttle it will feel like less power and when I let out a bit it will get a bit more power until I press back down. Then it does it again. Like I said it runs wow great while its cold. But once warmed up the engine light throws and begins to act up. Truck has a new map sensor, new engine coolant sensor. New thermostat. New plugs and wires. Fresh oil change. New fuel filter. Any help would be great. Please HELP!