I own an SR-3000E Ibanez 4 string bass. The action is perfect. The open strings and the 12th fret note are perfectly in tune. However, the first 3 or 4 frets register a bit sharp on my tuner. Then from about the 5th to 12th fret it's fine. Then from the 13th fret on, the notes seem a bit flat. Any ideas how this can be fixed? Is it possibly a nut issue? Thanks!!!!