You are positive the water pump is good, right? I attempted to test mine by pulling the thermostat housing off and moving it and the upper hose out of the way. I also took the thermostat out. Then, I had someone start the car and watched the flow as it came out of the block (only do this with engine cold and get ready for a mess...keep your face away from there, too) On my car, it was just a little bit of flow at idle, like a garden hose near its lowest setting just kind of running out of there with no pressure. Had the helper rev it a little and the volume did increase, but I still wasnt impressed.
Only reason I ask about the waterpump is because my car's heat would blow cold sometimes and warm at other times, but never really hot (Was only warm if the engine was spun up a little bit). Also, since the coolant wasnt circulating in the engine very well, it was boiling in the block and going into the overflow tank in the form of steam. If the cap was off, the radiator cap would purge the pressure every once in a while.
When I got my water pump out it had all of its plastic impeller blades chewed right off of it!