I'm sorry, I don't know how much experience you have on your tractor or what kind of conditions you're working so if it seems that I'm insulting your intelligence, pardon me.
On my Kubota, if the radiator, heat exchanger and oil cooler aren't kept spotlessly clean it can lead to it starting to overheat, the a/c blowing hot air and oil warning lights starting to come on. When mowing in dry, dusty conditions I have to take a break after about 4 hours and let it cool down, then take off the side panels and wash everything out real good. In fact, when that a/c starts blowing warmer air is one of the clues that it's time to clean her out. With small cabbed compacts there is just so much stuff crammed under the hood all trying to gulp air to try to stay cool.
That's where I would start but if that doesn't help, I'd go with Jim's advice.