Did you completely flush the system when you replaced the radiator?
Sounds like the stat is either gunked up or needs replacing. Sometimes just cleaning it will help, you might as well flush the system if you didn't already otherwise a clean or new thermostat will end up in the same condition in no time.
I blew mine out a couple weeks back (literally blew it out). Unhook one of the core hoses (top) and hook up a hose pipe with a funnel. Unhook the bottom hose from the Rad, let it drain, then blow down the funnel with all your might..... you wouldn't believe the crap that comes out doing that, crap that even a hot water flush didn't shift. My TJ heater now works a treat, and no over heating