Sounds like you are closing on the solution, Steve, congratulations.  Its
always nice when you get to the point where you can fly without one eye on
the temps.

The boundary layer (particularly with coolers to the rear of an airframe)
appears to play a big role.  The layer increases in depth as you go rearward
and it creates havoc with the operation of cooling ducts and diffusers.  One
article I read indicated the American P-51 and German fighter designers
figured out the significance of keeping the boundary layer out of the
cooling ducts whereas the British supposedly continued to be plagued with
cooling problems with the Spitfires due to their cooling inlets being partly
in the boundary layer.  FWIW

Keep us apprised

Ed