Was the original thermostat a 160 degree one? If it was a higher temp one, say 180 degrees, the 160 one would be opening up to soon and not allowing the rad to cool the coolant properly. Sticky thermostat? or something stuck in it partially blocking it? Clutch fan failed, if you have one?
Can't remember for sure, but I don't think you see much circulation in the rad until the thermostat opens up.
I think in your other post you said you where losing coolant??

Was the motor recently rebuilt?? You could try re-torquing the heads as a last resort.