Yesterday I was working on my XP-60, and a funny thing happened. While playing a song, I noticed 2 parts (the only ones playing at the time) were going badly out of tune with each other. And later in another song, one part transposed itself up by a whole step. The only way I could fix this was to start the song from the beginning, or to restart the synth. It made editing very difficult.

Any ideas why this is happening?