When you service the thermostat or flushed the system you did not let the truck sit there and idle to work all the air out of the system. It is really easy. All you do is take the reservoir cap off and let the truck idle. It is called burping. What I did to mine cause I had a similar problem was start fresh so I could eliminate these things was. Go to Ford and bought a new reservoir cap. There is a updated one, and it is very cheap. New thermostat, thermostat gasket. Just add some more coolant let the truck idle and burp and you should be ok.