My Prowler started a new act! The cooling fan comes on at 150 F, and it will cycle for the first 5-10 minutes of running. The upper hose heats up as normal, the bottom hose also heats up, but then (after 5-10 minutes) it begins to cool. I monitor the outside of the head for surface temp and it never exceeds 160. After the bottom hose cools I open the air bleed and nothing comes out! There is no trace of oil coolant contamination, no leaks in the system that I can find. I'm thinking a cracked cylinder head that starts to allow combustion gases into the coolant passage as it warms up. I would think a leaking head gasket would only get worse. I have a new water pump, and the thermostat is fine.