You either have a bad thermostat, air pockets in the cooling system, or a cracked or blown head gasket.
do a compression test on rad? check for leaks, also like rowefast mentioned above a head gasket..check it by performing a pressure test on each cylinder..if all cylinder pressure is or close to each other then head gasket should be ok..if in one cylinder you have a big drop, then you have a leaky head gasket
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Kelly Little Ashford