Toyota Corolla

I have a 2003 Toyota Corolla SE with about 56,000 miles on it. Sometimes when the car is sitting still either in drive or park the temperature rises, but doesn't go into the red. Usually when I start moving it goes down to normal range again. Sometimes I can turn on the AC while I am still sitting still and the temperature also goes down. While I am driving it I haven't had the problem occur. This issue doesn't happen all the time, only once in awhile.
What type of problem could this be? Any help would be really appreciated.
May 17, 2007.

Check your coolant level and replace the thermostat for a start.

May 17, 2007.