Sounds like a good candidate for service.

Assuming you don't have anything MIDI connected: sounds like stuck buttons. Probably needs a membrane (electrical) / contact (electrical) / button (physical/mechanical) cleaning. Although it's possible there's a logic board issue - a good electrical+mechanical cleaning would be the first step for aging gear. While open, it'd probably be worthwhile for the tech to take a look at the bulk caps for the I2C parts.

Service should be done by a qualified tech to minimize risk to the gear. For out-of-warranty work, I wouldn't necessarily be "pushing" Yamaha certified techs. Although a tech with lots of Yamaha experience may know the common failures and more quickly/accurately identify the root cause.