Did you bleed the air out of your cooling system after installing the thermostat? Some come with bleed valves on them but a lot do not. Use this link for instructions on how to get the air out if yours did not have the valve build in.
http://www.instructables.com/id/How-to-Burp-your-cars-cooling-system/
If you are not losing water anywhere then it either air in the cooling system or you may need a new water pump and or clutch fan, might be all one unit. I would refer to a haynes/chilton manual for that repair or a lot of Chrysler or pt cruiser forums have the factory manual available for download.