I happened to stummbled upon your question on Death's profile page. I just wanted to add to the trapted air idea. Air can be trapted within the heater hoses, the heater elements as well as pocket areas in the block sometimes. Simply burping the coolant hoses in the front can't always force the air trapted in places on the back of the motor or in the heater lines. The only way to remove all air from the system is to vacuum it out.
Shops have the machines for this job (their name is escaping me) and I would suggest looking into it.
Also, I don't believe Death has seen your response because you replied to him on your own profile page. He would not get a notification that you responded unless you wrote on his profile page.
Hope your issue gets resolved.