It's hard to say but first thought is inconsistent spoke tension or maybe just plain crappy spokes.
If you have a spoke tension gauge, or some other means of assuring all spokes are equally tensioned to an appropriate tightness, that would be a good place to start. If that doesn't work I'd rebuild the wheel with some quality spokes from DT or Sapim. 14/15's for DS. You might be able to get away with only doing the DS if money is tight.