This is what I was talking about in the previous post. If all the cracks are around the bottom corners of the drop down area more that likely the core is flexing pulling at the corners. Adding the new core will only help if your old coring is actually bad, if that is not the issue the only thing I know you could do is add tabbing under the box in all the areas that may be flexing and cracking. You could add some filets in the corners then lay up some glass over that to stiffen that area up. Just don't tie it to the hull shell.
