[Michael C Henry]

Check the thermostat bypass ,that short hose between the manifold and water pump.If that is plugged there is no circulation near the thermostat untill it is much hotter in the heads and the hot water causes the thermostat to open usually violently.I've seen the metal nipples rust and I found the hose to have a blister inside that chocked off the flow,but was not visable from the outside.