I have a 2002 F735, yammer diesal. Overheats after about 20 minutes of mowing, I have flushed and clean radiator, new hoses, thermostat. Only 50 hours on new motor. I have checked and not bubbling when running so not a head gasket issue. But have to let it cool down and add more antifreeze, mixed 60 /40. Its driving me crazy. HELP..