I've got the intonation set nice on three of the four strings on my Jazz bass. The E string saddle is all the way foward (towards the neck) and the open E is in tune, but the 12th fret is way flat, almost to D#. The first fret F is in tune, but the notes start getting flat as I go up the neck. What's up with this? I put on a new set of strings and no improvement.