I don't know if the S90XS has a "test mode" as some other Yamaha keyboards do, but you could try holding down a C# Major chord in the keyboard's second octave while turning it on-- that's how the test mode is activated on other models. If that puts you into the test mode, I'm not sure what buttons to press to scroll through the different tests and select them, but on other models you use the [+] and [-] keys to scroll through the list of available tests, press [START] to enter a given test. Other models that I know about have tests that check the keyboard's ROM, RAM, and so on. I suppose it could be a chip going bad, or maybe a loose connection inside. Your best/safest option would be to contact a certified Yamaha service center and ask them about it.