2006 ford freestar overheating on the highway

Second time doing this first time changed thermostat worked ok for three months. Once again this week noticed temp Gage climb to red zone on dash while driving on the highway.It only happens on the highway . does any body know what the problem could be. does a small leak on the intake manifold cause the overheating . coolant level was low as soon as i noticed overheating but has remained same level since a topped it off a dew days ago. could the thermstat be the problem again has anyone else had the same problem and was the remedy .

The thermostat was probably never the problem. You have a small leak somewhere and that is letting air into the system. In the process of replacing the thermostat, you bled the air trapped air out. Bleed it again, and it will be okay for a while, but you need to fix the problem.