I have had this issue off and on for several years now. I have done all the tests head gasket / intake gasket . Did have to put head gasket repair in it. Today i was driving and lost all power. Looked at the gauges and the temperature was buried on high. I quickly pulled over and raised the hood expecting to see antifreeze everywhere. The overflow bottle was completely full and bubbling so i figured it had air in the system . I let it set for 45 minutes on the side of the road. The level in the overflow did not go down any. I know that opening the bleeder valves with the temperature so high can just cause the coolant to boil , but i did open it a little and a ton of air came out . It did start blowing steam after several minutes which is when i closed it off, and just sat there another 15 minutes. I looked at the level again and it had completely dropped to normal. I was able to go ahead and drive it home and the temperature never went over the mark below halfway. I have been researching this since getting home, can there be a leak on one of my quick connects for my heater core causing this or am i going to have to try and do the head gasket? It has gotten very frustrating.