What is failing, the thermostat or the water pump or something else?

SOUNDS LIKE YOUR THERMOSTAT IS STUCK ALSO THESE CARS HAVE HIGH HEAD GASKET FAILURE SO DON'T KEEP DRIVING IT UNTIL YOU REPAIR

Agree with Global helper,thermostat may be failing, water pump seldom fails without leaking, radiator may be partially restricted, the viscous fan hub may be bad (definitely check that) also have a chemical test call a block test done, cheap quick test to see if the head gasket is leaking!