It is overheating as it steams up and you can see that water stops coming out of tell tale. when it first did it I changed impeller,then I took for a run still did it,so I changed head gasket,fitted a thermostat as it didnt have one,still same issues. it is fine on the muffs,at slow speeds it is fine,as soon as you start to get around half throttle or so, you can see the water stops flowing out of tell tale.then back off it starts pumping again and motor cools down again.I pulled head back off,there is no corrosion in head or block.