Generally speaking, buzzing on the lower frets means you need more relief. Buzzing on the upper frets means you need to adjust saddle height.
So- I would first check the relief on the lower frets (by fretting the first and 15th frets, and looking for a small gap under the 7th fret). Adjust the truss rod until you have a small gap.
If it's still fretting out, look at raising the saddles. But this is odd ... it shouldn't just change on its own. That makes me think relief might fix it, or your bridge is out of whack somehow. Assuming it floats, if it's resting in a different position (due to a change in string or spring tension, e.g. downtuning), this could do it.