Failed water/coolant pump? Happened to me.
Tractor will over heat if used for more than 15 min even if it's just idling. radiator is full and the radiator fan spins, screen is also clean.
Any other ideas as to why it's over heating? Has around 300 hrs on it.
Crank it up. Have access to the radiator hoses. When gauge says it's hot is the top radiator hose hot? How about the bottom hose?
If top radiator hose stays cool the thermostat isn't opening.
If top radiator hose gets hot but bottom hose stays cool then water pump isn't circulating coolant.
If both hoses are hot then gauge is faulty.