I understood this problem to be related to the mast
step. On many Nordic 40's the mast causes the floor to separate from the bulkheads in the area of the mast step. The problem is corrected by inserting a stainless steal plate under the mast to spread the load. I beleive they beef up the tabs in the area between the bulkheads and the floor as well.
I have a picture of the plate installed on a Nordic 44, but I couldn't figure out how to insert it into this post. If you really want to see it, send me a PM.
Hope that helps.