4.6 still overheating after all of this!?

I have a 2002 Ford F-150 that started overheating and it's got me stumped! I did a flush of the system as well as a new thermostat. I was good for 3 days and it did it again. I then replaced the overflow cap. Still no dice. What's funny is this problem comes and goes at random. I have found a way to temporarily fix the problem by releasing pressure from the overflow cap and squeezing the top radiator hose 4 to 5 times. Seems like a blockage right? So I replaced the radiator(I also "burped" the system). It was doing good until the very next day. I started it up and drove about 5 miles and it did it again!
There is no water in my oil. I don't belive it's the head gasket. Like I said, It happens at random. Does anyone have any suggestions?
Thank you.

Walter Erickson
June 2009