Put in thermostat car runs hot leave out fine.

Without the thermostat, you would not be able to get the heater to work correctly. Thermostat should never be removed from the cooling system. If it is overheating, something is wrong and taking the thermostat out is not going to solve the problem, it would only delay the overheating, unless the thermostat is bad.

Are the coolling fans working when it overheats?