Do you use elastics to keep the boards in place? How do you rig them (pulling in what direction)?
If you rig the elastics so they pull the top of the boards forward, you are reducing their angle of attack for vertical lift, making the boat more capsize prone.
Try rigging the elastics so they pull the top of the boards back, which increases that angle of attack and, consequently, the vertical component of lift.
The more you need vertical lift from the boards, the more you want the top of the board pulled back. Enlarge (sand) the cases in the fore and aft direction if you feel that more vertical lift is necessary.