I had much the same problem some years ago on my Alesis QS6.1 synth. It would output midi pitch changes on it’s own. The solution to the problem was to replace the pitch wheel potentiometer. If you want to investigate this possibility, I would try cleaning the pitch pot first with some potentiometer cleaner like Deoxit. Be careful however. There are different kinds of Deoxit for different purposes. If memory servers, you want Deoxit F, or something like that. Sorry, I’m not near my tools at the moment.