Do you have a temp gun? When the tractor's high temp light comes on (if it has one) or when you think it's getting too hot, check the temperature of the inlet (top) and outlet (bottom) hoses to radiator and see what the differential is across it. Some people might try to feel hoses with their hand, but I'd recommend against this for a number of reasons.
If your tractor's temp light is on, it's probably pushing at least 230 deg F on the water to the radiator and if the radiator is working as it should, I would anticipate the outlet hose temp to be no higher than 180 deg F. Now I don't know what the actual design conditions call for, but if the temp is 215+ deg F coming out the bottom hose to the pump, then I'd have to say the radiator is the culprit.
This is an easy check and temp guns are getting fairly cheap these days and quite handy on all sorts of projects.