Overheating engine

The engine overheats and air gets into the coolant system and I cannot find a solution. To date I have had the thermostat changed three times, a new radiator was installed (that was not needed), the head gaskets have been done (only to find that was not needed), the water pump and timing belt have been changed (not sure if the water pump needed to be replaced), the problem still exists and I have had three mechanics do work on the engine to the tune of a lot of money. At idle the temperature gauge stays around the middle, under load or driving a little while the gauge goes up almost to the red. While driving and I turn the heat on in the car, the temperature gauge will go down to about 3/4. The water system boils and flows out the overflow container, both fans will kick on when hot and I am hoping the thermostat opens. There has been two pressure tests done on it and no leaks detected. As I mentioned I am in the hole for $1,700.00 so far and no change to the problem. My current mechanic is a great guy and he has put a lot of his own time into this repair as well and he feels bad that he cannot solve the problem. Any ideas as to what the mechanics may have missed?