Radiator overheating problems F-150

My neighbor has a 1998 Ford F-150 that he replaced the water pump and thermostat. He was then having problems with overheating. I'm not sure why he replaced the pump in the first place, I assumed it was leaking. The truck would overheat very quickly while driving at 35-40 mph, then cool very quickly at idle. The colder the outside temp, the worse the problem was. We finally determined that the late model 98 didn't have a water temp sending unit, it has a head temp sending unit.

That was on Sunday. I checked with his brother later in the week and he told me that the problem was fixed - air blockage somewhere in the system. I've heard about this problem but never experienced it.

Any information on the subject and how to avoid it?