It would definately help if you detailed exactly what you are building; i.e. - product, thickness, material type, etc.
Just by your short description, im assuming some box type (cube) product, that is made from thin material. It seem sensible considering you are talking about "corners" and in between weld ripples. As mentioned, it does sound like a technique issue. Corners can be a toublesome spot for many welders. As far as in between the ripples, is this where possibly the welder started and stopped his beads? Lack of penetration or lack of fusion, maybe?
If you are having problems with leaks, why not try to implement an inspection plan prior to the testing?