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When you have the IR gun handy, the engine running, check the inlet and outlet temperatures of the radiator to see how much temperature differential there is. You can also move the IR across the front of the radiator to see how even the cooling (or lack of) is.
A radiator can look good, flow test good, but still not cool properly due to scaling inside, or the fins being loose on the tubes.
BTW, has the radiator been painted? If so, what kind of paint? Radiators require a special kind of paint for heat dissipation.