This car overheats after twenty minutes but only in the heat what could it be
Low coolant, coolant pressure, or a bad fan come to mind. Assuming the AC is running.
I had that same problem, my thermostat was stuck.
Nobody can tell from here. Only possibilities can be suggested.
One thing for sure is that if it really is getting too hot then each time it happens threatens and or shortens the life of the engine.
Get it diagnosed and corrected before driving this car.
Northstar motor? Headgaskets.
What year, what is the mileage, and what is the engine?
I would check the thermostat first AFTER making sure the coolant level is good.
One more possibility, air trapped in the cooling system. In anything related to the cooling system was repaired soon before this symptom appears, definitively should be considered.