2004 Pontiac Grand Prix

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.
June 18, 2010.

Is the cooling fan shroud in good shape and are both cooling fans coming on?

Jun 20, 2010.