Why is my car overheating?

Do you have the origninal fan shroud in place?
Have you replaced the radiator cap?
Have you done a pressure test to the system?
It could be a bad head gasket.
How are you determining it is overheating? The stock gauge could be bad, or sensor, if that is all you are using. Get a $20 Infrared thermometer.
Sometimes taking the thermostat out will not slow that fluid down enough for good heat transfer.
It is like if you want to cool off a piece of pizza. Too cool it off blowing across it quickly willl not work as well as blowing slowly across it.
Get the thermostat, unless you have used other means and are experiencing steam overheating, and get back to us.

You can performa a cyclender compression test anad a leakdown test amd/or a coolong system pressure test.
A quick cooling system pressure test it to ggrab the radiator uppper hose when it is still warm and see if it has pressure. Ususally if perfomrance is an issue, there is a bad head gakset, fuel managemnt, spark or igniton and air intake issue.
With the issues you describe, I would first perfrom a compression test and then performa a coolling system pressure test.
You can also drain the oil and there are some service station that have, "Sniffers" that can detect anti-freeze in the oil.
Try that first.

There is a tester, (that is not too expensive and a lot of parts store chains will let you borrow), that has an addapter that goe sinto the spark plug hole. You will need the pressure specification from the manufacturer or just get a Haynes manual at a parts store.
The tester has a dial gauge at one end and the sprak plau adapter that thereads inot the plug hole one the other end.
It is best to turn the engine over by hand, but read the directions and see if it is ok to turn it with the stater.
It will give you the compression made by the cylinder. If it is leaking it will be low.
I hope this helps you out. Take care.