is the car boiling the coolant out?had a similar problem but only ran hot under a heavy load.the problem was grout had built up in the passage above the thermostat which would be drivers side head,all the way forward. you can either remove the head, or what would you was removed the thermostat and put air pressure to it.it has been over 9 months since we did the repair and I still talk to the person with no problems as of yet. The answer was given to me from an actual Chrysler technician. Hope this helps