Thermostat replaced in March 2014. So I don't know why the temp is rising when I sit too long.

If you have topped off your radiator fluids and it still over heats, I would do the following: Purchase air bleed kit to purge all air from the system, replace the radiator temp sensor. Make sure fans are kicking on. If not, may be the radiator temp sensor.

Thermostats are getting cheaper by the year. There is a large portion that do not seal 100% when new. I always test them in boiling water to ensure that they fully open and completely seal after cooling just below their opening temperature. I have some cars with the t-stat in horrible spots and do not want to replace a part that cheap twice. Of the last 30 or so 3 new ones that were closed when removed from their box would not seal after testing. It does take much fluid passing through the T-stat to heat up your vehicle in certain situations. If the T-stat was just changed it might be an airlock but since it was done in March I would lean towards a faulty T-stat or a newly developed leak. Check the actual level in the rad when cold and top off if required. Your engine, rad and other components of your cooling system must be full - no air. Then add additional fluid to the hot or cold line in the reservior. If you are low in the rad top it off then check it and the res. again to ensure you are not loosing fluid.

Thank you guys, I have put water in my radiator (until it bubbled back at me), and it hasn't ran hot or gotten close to half way point.