I would suggest taking the car to a different shop for another diagnostic opinion; obviously there is something wrong with it, even if the first one could not find it.
You could be helpful to teh technician, if you could take notice if teh fan is running when it is overheating, and if tehre are any specific conditions under which it overheats such as when the AC is turned on.
The technician should be able to duplicate the symptoms to be able toa rrive at an accurate diagnosis.
Possibilities for an overheatig problem include:
(1) Cooling fan not operating correctly or a control circuit (computer, temp sensor, or relay) problem causing the fan to not come on
(2) Cooling system which contains too little antifreeze and too much water, which lowers the boiling point
(3) faulty thermostat
(4) Low coolant level, from such causes as a leaking intake manifold gasket on a 3.1 engine or a small radiator leak
(5) Failing head gasket, allowing exhaust gas to entera cooling jacket
(6) Bugs, dirt, or other debris restricting airflow through teh radiator
(7) A radiator with internal deposits restricting coolant flow through the core
(8) A failing water pump or corroded impeller, causing insufficient coolant flow
I hope this is helpful; if so, clicking my accept button would be most appreciated! Thanks!