I had a similar problem with an early ducted fan called the Skyburner.

I solved the problem by installing counter-rotating vortex generators and a wing fence. Seemed to work very well. The wing fence keeps the span wise flow from traveling to the tip. The vortex gens act much like the turbulator strips.

vortex gens are used on B-737, A-4, Learjet and many others. Fences on the learjet, mig etc

I just made up 3-sets for each wing, forward of the aileron, about 6" total length, set at about 30^ off the cord line. / \ / \ / \

here's a picture of the wing. The clear vortex gens are in the checkerboard.

Jack