93' trans am

i have a 93' trans am and when im driving the tempature stays cool, but anytime im stopped for like 5 minutes it tries to overheat up to 240 degrees. does anyone know what is cuasing this? any suggestions would be greatly appreciated, already changed thermostat. thank you.

I once had that problem and it was an overlooked weak hose. Are you losing coolant? Maybe even the thermostat you put in, could be in wrong. ( it can happen)

If it were me I would take it to a garage and have them do a pressure test, and check for small leaks you can't see with your eyes..most places will check for free.