Here's a long shot. I have a Rhodes Electronic Piano, which is NOT what you think of when you think of a Rhodes. It's basically a rebranded ARP four-voice piano. Pretty cheesy sounding.
I've found that the upper two octaves or so exhibit a strange symptom. Notes struck there have a ring mod type effect going on and when the two highest octaves of a given note are hit at the same time, it actually plays the same note but the next octave lower.
I'm curious if this is evidence of a problem that might be general enough to electronic keyboards that someone might have some knowledge of it?