Sounds like either the radiator is being blocked of airflow, or the water pump is worn out and is pumping the antifreeze in and out of the radiator quick enough.
Here's a simple test...
While sitting still (as if you were in stop and go traffic) then hold the gas and rev the engine up to about 3500 RPM. Keep holding it at that speed while sitting still.
Watch your temp gauge at the same time... If it still rises and attempts to overheat... Turn it off ASAP you need a new radiator.
If it does not over heat and runs fine... then the water pump is the culprit.
Good job on replacing the thermostat, that's the first step and it should be replaced to prevent overheating anyway.
Goodluck email me if you need more advice I'm very good at diagnosis.
-Mike.
