My mustang at an idle will get really hot. I'll let it idle for like 15 mins with the A/C on and it'll get really hot, almost to the red. The electric fan works and I changed the coolant. Then I'll drive it and it'll stay hot for another 15-20 mins then it'll slowly drop a little.
What could it be?
And after you've topped off the cooling system with a proper 50/50 mix of Anti-Freeze and DISTILLED WATER, you should "burp" the system by squeezing the upper radiator hose and progressively working the air pocket out towards the radiator... with the cap removed. You'll see the air bubbles exit out of the coolant tank.
I'm going to say thermostat as well but it could also be a water pump or head gasket issue. Keep an eye on the coolant level and check the oil as well, also being into the off road scene I say check to see if anything is blocking the radiator fins, dirt, trash, dead animals, ect.