It’s a fairly common ailment of old Japanese (and some american) keybeds: the guide bushing for the keys side walls shrinks, deteriorate, and crumble. The obvious sign will be when you play a key and it feels like it is getting caught on something on the way down, and has a lot of side to side sway. These rubber key guides are a direct replacement for your old ones, and will provide a greater playing experience.
Available in 61 key sets. If we missed another keyboard size, let us know!