Bill,

-one possibility

The air coming in from your 3 inch duct may actually be leaving through the NACA scoop during slow flight.  NACA scoops are somewhat sensitive to orientation to the airstream and while in slow flight the orientation may be off far enough that the NACA scoop is no longer effective.  Two parallel sources will fight each other if the pressures aren't well balanced.  In this case, if the NACA scoop pressure drops all of a sudden, you would see a flow reversal. 

If you measure the pressure ahead of the cooler as you slow down, you should be able to see this transition occur.

 

Chris Zavatson

N91CZ

360std