Its best to identify the problem before you try and find a solution.
Heres how to test the factory thermostat...
I would reccomend you try limiting the airflow by covering up a couple of inches of the radiator as I previously mentioned, if it doesnt sort the problem, the remove and test the thermostat. If it does stop the problem then you know I was correct and you radiator is too efficient for your setup takign too much thermal energy out of the water. If it reduces the problem then cover a little more up and see if its any better.
A Sytematic approach is best.