My Case 1840 with 1500 hours overheats after about 10 minutes of low speed operation (at least the idiot light comes on). When this happens, the top of the radiator is pretty hot to the touch, but not boiling. The bottom of the radiator and lower hose remains cool; feels like ambient temperature. Water from a garden hose runs through the radiator with no problem. The water pump turns fine and the impeller looks like new. I also suspected the thermostat, but seems to me the upper radiator reservoir wouldn't get hot if the thermostat wasn't opening. There are no bubbles in the radiator reservoir as I have seen happen with a bad head gasket. Any ideas would be greatly appreciated! Thanks, Gordon