This code indicates that the engine cylinder head temperature sensor indicates the truck is overheating and going into a fail safe strategy. IBelow is a list and description of the trouble code. You need to first check the coolant level, if it is low then look for a external leak somewhere. If the level is ok then it could just need a thermostat replacement. If the engine actually does not even seem to be overheating or hot then it could be the sensor giving it a false reading.
P1299 Cylinder Head Over Temperature Protection Active
just behind the alternator on the intake manifold on the 2000 5.4L engine: