I think much of your problem is due to increasing the chord and thus the wing area. Doing it the way you did also reduces the effective tail length. Each of these side effects are destabilizing on their own. Added together and it's quite possible to end up with a serious case of bad stability.

Also without seeing your final airfoil shape it's hard to say but it may be thick enough or oddly shaped enough that the airfoil itself is causing some additional issues.

Wing warps would obviously cause issues. Also any uneveness in how the ailerons meet the wing can cause issues.