tnmower

I have a 2 yr old Craftsman 18 HP riding mower with the same tranny issue. Works great on flat ground. After about 30 minutes of mowing, it starts whining as I go up hills. Eventually, it loses all power. If I return to flat ground or park it for about 30 minutes, it will begin working again. Makes for a long day of mowing. I have replaced the drive belt with no success as well as changed out the oil in the tranny oil reservoir. Looking for any advice as well.