I've been on a quest to figure out what to do for a few months now. I have a Yamaha DGX 305 and an octave in the bass side has sounded awful for awhile now. It has also progressively spread to a few more keys and is heading towards middle C. It started out only I could hear it, but now it is strongly out of tune with the guitartist. I have done a master reset many times. Sometimes it will temporarily help, but after a few songs it is out of tune in that octave again. It is set at 440. I have researched this into the ground and have found nothing, but "do a master reset" This is not remedying the problem. Anyone have any ideas or have you encountered this?