Why does my neon stop running above 70 degrees

Why does my neon stop running above 70 degrees Anytime the temperature reachs 65 or higher my car seems to overheat. After 15 minutes it runs again til it heats up. I have changed the thermostat and had the radiator flushed twice has not solved the problem. When it heats up from a stop it hesitates and almost kills then runs rough