heavier spined arrow is what solved all my issues. I was shooting easton 400's - the chart on the box said that was the correct spine for a 27.5 inch arrow at 66lbs. draw weight.. well i ran into the same issues as you. Bought some 340's, and immediately saw a huge difference in broadhead flight, while field tips still shot the same as before. Just my .02 cents. Its something to consider at the very least. Do you have access to other broadheads that you can test, maybe a buddies or something?
Ive been there, its very frustrating!