[MikeK]

It is also possible that your head gasket leaks exhaust into the water jacket when under heavy throttle. The bubbles will cause an expanded volume that goes out the overflow. Have you retorqued lately?

There are also several ways to check for this- electronic radiator sniffers, chem test kits/ acid strips, filling the rad to the top then pressurizing individual cylinders with shop air through a plug adapter and watching for a rise in the rad H2O level. A compression test that gives normal model A readings does not mean the gasket holds up when subjected to cylinder firing pressure.