Possibly you have some frets that are actually moving up and down within the groove cut for them. When you are leveling you are applying pressure on them ie pushing them down as you level and they are indeed level to the pushed down position. When you go to play those frets that aren't seated properly they spring up and create the buzz when you fret above that position. This happened to me once and I chased it for weeks doing just what you have done. I finally identified the problem by pushing down on each fret and noticed movement in a couple of them. The fix is to remove the offending fret, fill the slot with super glue and recut the slot. I hope this helps.