The chinese Farmpro we have at the family farm has this issue. There's nothing wrong with it other than it's just chinese junky quality. But what i've found is that with the 5ft finish deck in high(er) grass it requires more hp than you think. And in my case it gets hot. When I use the 4ft brush hog on it then it's fine all day long no matter what the grass or weeds look like. This is on a 20hp 2cyl diesel Farmpro 2420. On my Ford 1900 at home I only use it with a 6ft finish deck and it too will heat up on high density grass. When the grass is thin it doesn't press the tractor as much. One other thing I've found other than regular cleaning of the radiator is to run a flush through the Ford and then clean water several times every two years. This along with some Water wetter seems to help it out a lot to keep the temps down.
Steve