The way I understood it is that only cold coolant was being spewed out. Sorry.
I would still go with the thermostat as the least expensive fix at present.
Has it ever been changed that you know of ?
How much coolant do you have to add after the resivoiur settles ?
Because it kind of sounds like you may have a head gasket going. Pull the plugs and seee if any of them are wet with coolant. If not you can pressurize the cooling system then pull the spark plugs, crank the engine and see if any water comes out of the plug holes.