My guess air lock?
When I had the farm (100) years old with old radiators that kept the house toasty warm...your right, they get vapor locked. There is an area on each radiator that every now and then we had to bleed the air out of each radiator. You could hear the air coming out of the radiator, when it quit you shut off the (stop cock) or whatever you call it...Also one time the water tub in the attic froze up and the furnace meter went to red, scared the hell out of me, called hubby at work and had a guy come out to check the furnace. He found the water source frozen and no water to circulate through the radiators. Actually had one copper pipe hissing out air..