I have a feeling that what you are seeing may be the product of a leaky head gasket as well. While you may not have a coolant leak into a cylinder, you may be having a combustion gas leak into the water passages. The description of gurgles may be telling the story. Combustion gasses are under a lot more pressure then what a radiator cap allows, 14-16psig vs 150/200psig under just crank speed, much more when firing and if it is pinging, a whole lot more, so it can be a one way leak. "Way back when" Prestone used to have that kind of a problem as well.