Foton 404 with about 450 hrs. Engine runs good and everything works good for about an hour of mowing. Then the ground speed will begin to drop while engine stays the same. Usually work in Low 3rd or Low 4th when finish mowing. By the time I get it back to the barn it is barely moving. Once it rests for an hour or two its ready to go again. Anybody got an idea of where to start?