I've built two of these, (just finished building one for a friend) so the problem you're having is fresh in my mind because I had the same issues. I solved the issue by doing two things.....1) building a wooden frame on the inside of the speed brake to stiffen it up and more importantly 2)..I added a 3rd hinge in the middle and it absolutely solved the problem for good. (I assume your kit is the same as the Comp-ARF Rookies)

If you wish I can send a photo.....PM me required.

Good luck with it....Ron