What are you topping the system off with?
What I think you're seeing is steam, as in water generated by the cat that's flashed to steam as the temp comes up in the system.
Does your oil level rise? What color is the oil? Is there any funny looking goo on the oil cap?
Fill the tank to the full mark when HOT. It should drop about halfway between full and add when cold.
After that, check it cold, after it sits at least 8 hours, if possible.
Do you smell hot coolant when you stop at the end of a drive coming from under the hood?
Look around the engine compartment, the Type 2 will leave a tell tale blue to whitish crust wherever it leaks. Pay careful attention to the radiator, top and bottom, all hose connections, the water pump (behind the pulley) and the heater control valve near the firewall.