Hi,
I had the same problem several years ago. It has to do with a rubber problem with the old Yamaha keyboard. I brought my Korg to a repair shop and they repaired the key that got stuck and revised the rest of the keyboard. It had something to do with the rubbers which were too long. It was actually I known problem with the Korg 01/w's of the first couple of releases (until a new keyboard was inserted). It costed me about 50 dollar and I haven't had any problems since.
Good luck,
Jochem