You may have an air pocket within the cooling system. Have you bled the system yet? There's two screw valves on the top hose leading to the thermostat that you have to slowly open when the car is running to allow the air to escape. Once you get a steady, but slow stream of coolant it should be good. I would suggest still flushing out the heater core, too. The deposits inside are going to take some time to break up and flow out.

I also know what valve you're talking about leading from the water pump to the heater core. It blew out on me and I had coolant pour out on me once early this year.

http://web.camaross.com/forums/showthread.php?t=230994

I picked up the flow control valve at my dealer. I don't remember it being so expensive. It comes right out and you need hose clamps to replace it. No need to cut and splice.