Hello, I have a Carvin LB 75 (5 string) that is 13 years old that I am very fond of. I have noticed when tuning that my open strings, when tuned perfectly, do not match my fretted notes, which turn out sharp up to the 5th fret, then flat on up the neck progressively. For example, open A is tuned perfect. When I fret the first fret, the Bflat is slightly sharp. Still on the open A, I fret the notes one by one up the neck. When I get to the fifth fret, the note (d) is perfect. As I continue fret by fret up the neck, it gets slightly flatter each time. By the time I get to the 15th fret on the A string (c) is VERY flat. And everything gets flatter as I continue up. I feel my intonation is almost perfect, as my open string and my 12-fret harmonic are almost perfect. It did have a new nut on it, could that be what's causing the problem if it was installed poorly? Please help! Driving me nuts. Thanks in advance.