Car won't start when outside temp higher then 75 degrees

My 1996 Cadillac Seville sts. Won’t start when the outside temp reaches 75 degrees or higher. If the car is being driven when the out temp goes past 75 degrees, it will start running rough. If you turn the car off, it won’t restart until outside temp goes down to around 72 degrees and lower. No warning lights or indicators read out. The temp gauge read normal operations. Is there a sensor or something that’s reading the out temp as the car is overheating and shutting it down?
2nd problem. Car will run fan on hwy but, will over heat when in stop and go traffic. I can't find any leaks once car over heats it won't restart for 3 to 4 hrs later. Replace thermo stat and coolet level is fine car will take about a qt after overheat.