I know you don't believe, but I'm sticking with my purge idea. I don't care how fast your pump is, if the bubble is in the system, its in the system. You said,
when a little bit of air (~5 ml) builds up it just gets shot out.
Shot out where? Its a closed system is it not? Air shouldn't "build up" for the same reason. The problem could be the block, there is a reason they came out with the GT block. Perhaps its not as good as others.