2008 A4 2.0 quattro, 113,000 miles...

I have a problem that is driving me insane. I have low or no heat below 40 degrees. I narrowed it down to the heater core getting air locked, but after bleeding it over and over, and even getting it flushed I still have same issue. The engine is heating up fine, but I don't get circulation through the core unless I slam it down on the highway and get revs up to redline.

Engine temp is fine so not thermostat.

Was hoping the leaky coolant flange was the fix but air problem still exists.

No other visible leaks.

The problem is an exact replica of the TSB on the j-plug in the 1.8t but I don't believe the 2.0t has the j-plug.

It it stays above 40 degrees I have perfect flow and heat. When it's cold it seems to be sucking air into the system and I am totally baffled as to where this leak could be, since I have no fluid loss.