The thermostat may be bad. I’ve had new ones fail, especially the fail-safe variety. Try running it without a thermostat and see if it boils over after shut off. If it won’t get hot enough, restrict air flow through the radiator with a piece of cardboard. Also, how old is the radiator? It may be plugged with the rust and crud. Insert a long, stout wire into the passages accessible via the cap. If it won’t reach the bottom tank, the radiator needs cleaning and rodding out.