I am having trouble with the underside of some shelled parts.  I have a part that is created by a loft and then shelled.  The surfaces on the under side of the part have some distortion, and it is causing problems for creating other features in that area.  I can not create ribs or cuts that extend through the troubled area.

Any help on what is causing this? Is there something that will cause a file to become corupt that can not be fixed?