Been having this issue since last summer. I replaced thermostat 3 times (thinking a faulty one), fan does come on, checked for leaks with dye and UV light (only place I saw dye is out of overflow), did a chemical flush today...still no change. Only an issue when it gets over 75...then temp rises...I turn on heat to bring it down to normal and have to run with heat on. Not fun when it is heat index of over 100 out! Suggestions on what to try next?

Sorry...didn't state what vehicle...2002 Chevy Silverado 1500 5.3 liter 4x4