Engine Cooling problem
2004 Pontiac Grand Prix 6 cyl Four Wheel Drive Automatic 150000 miles

my issue with my grand prix is the temp is rising into the red, especially when my a/c is on and im driving on the street. i thought it was my thermastat i changed that two days ago and today it went into the red and when i cut the a/c off and the heat on it goes down.....please help i dont understand and i dont wanna damage anything by driving....by the way it doesnt do it everyday just on hotter days...