99 f250, v10 auto, pulling 4k pounds at 75 mph. cruise set at 75 and air cond. ON.......check engine light came on and put me into limp mode.....seems it only does it when the air is ON.......on the way home from our camping trip we were flyin along great and as soon as I hit the air, CEL on and limp mode again.
so what the heck is it, in simple terms. I carry my code reader, and if I shut it off, clear the codes, it runs like new again. my code reader says "no definitions"
thank you so much guys.

 

P1299 - Cylinder Head Temperature Sensor Detected Engine Overheating Condition

If your engine isn't overheating, it's most likely a bad sensor.