Overheating

Have you checked if the radiator fan is turning on?

The best suspect is a leaking cylinder head gasket. There is a tool to perform a chemical test for that. It involves drawing air from the radiator, while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are sneaking into the cooling system, the liquid will turn bright yellow.

When that air sneaks in, it can pool under the thermostat, preventing it from opening. Thermostats must be hit with hot liquid to open. Hot air will not do it.

You can find the tool at an auto parts store that rents or borrows tools, but they will make you buy your own bottle of fluid. It becomes ineffective if it is contaminated with coolant, or if it freezes. They do not want to risk having you contaminate their fluid, then they borrow it like that to the next person. Due to the cost of the fluid, it might be less expensive to have your mechanic do this test. It just takes a few minutes.

Just to add to it, if they put stop leak then I would suspect a plugged radiator.

Dandy point. Wish I would have thought of that.