I know you probably bled the air out of the system, but sometimes it is just very difficult to get rid of it. One old trick is to drill a small "bleed" hole in the thermostat. Then after running it to temperature, make sure the coolant is at the proper level (after it cools a bit of course). I, and many others, have had the same problem, worked for me and them. Since you tried so many things, this may be something to consider trying.
don
