The coolant must be in contact with the inside radiator surfaces in order for heat to be transferred through the radiator walls. If the fluid is moving too fast, the thermal energy does not have enough time to transfer enough heat from the coolant through the radiator to the surrounding air.
A non-contact IR thermometer can be used to check radiator inlet and outlet temperatures. About $50 at Sears.
FWIW, make certain that the fans are working.
Steve
