Was it doing it before the thermostat replacement?
If it wasn't, check to make sure the thermostat is installed correctly. Sometimes when the cover is getting put back on the thermostat can shift without noticing and it won't work properly.
If it was, I would say start by doing a very thorough radiator flush. If that doesn't fix it you probably need a new radiator.
From the sounds of it, if the radiator is staying cool, you probably have a blockage somewhere.
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