Sounds like a lose or bad connection in the cups from what you are describing. If in warrantry (or out of warrantry and unmodded) I would contact grado. I believe they have a pretty reasonable service fee for out of waranty repairs as well. If you have modded them then you should know how to get into the cups, where I would check all the connections, and resolder them as needed.
9/6/12 at 11:45pm