i have had this before, i think/hope it will sort itself when the car has been driven a bit , i am sure there is a bit of air somewhere
Well i changed the thermo and the bottom hose coming out the radiator was still cold, got the temp gage to stay at 90 and blow hot, the odd occasion it blow cold, the pressure built up in the hoses and when i unscrewed the cap of the expansion tank the water flooded out, there was deff no air lock as we got it all out. The pressure was building up due to the head gasket as gone, so new head gasket to be done tomoz, plus redoing the valves and seals etc. Then I would have rebuilt the engine. lol. Glad I got a very good mate who is a mechanic and very cheap. Thank god hehe.