Shern is spot on about the temp differential across the radiator; the water flow rate is so low that a great deal of heat is being exchanged. More than is intended.

Why? Again, as Shern noted, the thermostat may be at fault. The thermostat is the flow control valve for water flow.
Two other possibilities: 1) An unintended restriction of flow in the system (the thermostat is an intended restriction), such as pieces of an old gasket, or a rag. 2) As noted above (post #5) the pump function may be insufficient.