Foamy, is the air output good, just not warm enough? Do you have an air compressor where you could alternate between flushing with water, and then air?
Bill, I see that he flushed out the core - but don't see anything about flushing the coolant. With the actuators turned by hand to full heat, that rules out anything with the controls. I've had cores that would pass enough to warm the hoses, but not the air. If you have other ideas, post 'em up - because I'm running out of them.