[Fasterthanyou]

Sounds like there is just a lot of air still in the system. It takes a WHILE for the system to self-purge. Sometimes it'll never fully purge. When doing the heater core, why not fill it up with water? Then run the car with the radiator cap off and watch for bubbles. When the thermostat opens up you'll notice the level in the radiator go down a bit, this is when you top off the radiator and wait until no more air bubbles come up. That's the best you can do sometimes and do NOT fill the overflow tank. That tank is there for expansion. It's safe to fill it up to the cold line but don't go filling it up past that.
Another trick is while the engine is running, squeeze all the soft rubber lines. The stock heater core hose routing always traps air (tube goes up, then down, then up). Good luck.