Strange confusing zs 120

please help I've got a zs that's nuts it started overheating randomly so I thought thermostat changed that it carried on so had the head gasket checked it front of me their was exhaust gases in the coolant so I changed the head gasket had the engine and rad flushed engine rebuilt with a full service and belts etc and guess what it still does it I don't know where to go from here I know the water pipe that leads from the bottom of the rad round the back of the engine to the thermostat is a lot cooler than the rest of the pipes is it possible another faulty thermostat? Don't know I'm losing my mind and my money please help any advice on where to go from here?

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