2005 Chevy Duramax losing antifreeze

My truck is losing antifreeze. It seems to push it out over flow jug. I have changed thermostats. This help temp but not problem. Any ideas would be helpful. From what I have found this engine is reliable.

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http://www.dieselplace.com/forum/index.php

Thanks I am just about sure I will Have to pull heads and Find from inside to out side. I have looked and it seems to be Head or EGR cooler.