You shoulda had that head rebiult or replaced. Those heads are notorious for cracking. Most times you won't see the crack without a MAG but grows alot with heat. Pull the No one plug and look for it being steamed cleaned from the water entering the system.
You can drive it with the Rad cap loose and see if it still loses the same amount. Reducing the pressure inside the system will often slow the leak.
Replace the cap too. They do go bad and push coolant out the gasket.