Off the top of my head, this is the best thing I can suggest for the moment. I'm not sure if it makes any practical sense though:
Perhaps check where in the coolant circuit the heater connects to the engine water circuit. One possibility is that you could be drawing coolant from the cold side of the radiator, through the heater core. Conduction/convection could be heating up the static water in the core, then when it opens it may be flushing through colder water. The more I think about this though, the less I think it is likely, would take some strange plumbing to do this.
The other option is, perhaps check your head gasket with a compression test. If you have a crook HG, you may be pressurising the water jacket with air/steam and as the heater connections are high in the block it may be forcing the water out of the heater core. A mate's heater core actually ruptured in his KE36 and filled his cab with steam.