I agree with Marlin, it may be a fan clutch failure or very weak. You need this to pull the air through the radiator. Did you check it yet?? I know this because mine was starting to run higher than normal on the temp guage and when I replaced the fan clutch, fixed it!!
Here was my thread on the fan clutch help I asked for FYI:
http://www.vehicross.info/forums/sho...ght=fan+clutch