Howdy all, I have a Yamaha RBX 765 and the low B string is acting funky. all the strings are intoned fine all the way up the neck, but the B string is only a B when it's played open. as soon as i fret it gets sharp. the harmonic is a B in tune, but the 12th fret is about half a semi tone sharp. i just tried loosening the screw and moving the bridge thing it sits on back and forth, but to no avail. these are factory strings. any suggestions?