I bought a chrysler 2002 sebring convertible with the 2.7l motor. I got it from my mechanics brother. At the time I was told that the car had a leak and need a thermastat. They put the thermastat in but it kept over heating still. Then I was told that I needed a water pump cause it was leaking from there. They put a water pump in and its still running hot when I drive it. Then he said to buy a new radiator cap, I did that and its still running hot. Then he said that I had air in the radiator and he bleed the system. Gues what. ITS STILL RUNNING HOT. I can drive the car for about 20 minutes, the fan kicks on but it still runs hot.
Have you been losing coolant? If so, suspect a leaking head gasket. If not, check the radiator to see if the cooling fins are plugged with bugs or debris, or if they are corroded. If they crumble like a rotten chocolate chip cookie, they aren't transferring heat to the air. The clue is the lower radiator hose will be as hot as the top one but not much heat will be blowing from the fan.
To check for a leaking head gasket, your mechanic can check for the presence of combustion gases at the radiator with a special tool. You could also add a small bottle of dark purple dye to the coolant, drive it a while, then check with a black light, especially at the tail pipe. The dye will show up as a bright yellow stain that you can follow to the source.