Having recently gone through this, your symptoms sound very familiar. I'd look at the overflow tube. I roto-rootered mine open again, or removal has been mentioned.
The cap seals at the rubber inner layer, against the "shelf" that is about an inch deep in the radiator neck (below the drain tube), but is designed to vent at a given pressure. This vented air/coolant goes to the space above the ribber, below the top of the cap, and is open to the overflow tube. If that tube is open, there can be no pressure in this area. For coolant to leak out the rivet (which does not extend all the way into the pressurized area and therefore is NOT sealed), or around the edge that is visible with the cap installed (which is a bellville spring type arrangement, snug fit but again NOT sealed), there has to be a restriction in the overflow tube that allows less flow than what is coming out of the radiator. Engine temperature (and therefore volume) changes just don't happen that fast, the tube should be adequate to drain that area between the top and bottom of the cap if indeed it is fully open.
