I have a Yamaha S80, which was the king of the hill until the S90 came out.
Short of disassembly or sending it off for repair, I'd get a brand new can of compressed air (one that has the little straw coming out) and blow it all around the key. Seems unlikely that this would solve a "sticky" problem, but if it were me I'd be trying this easy step way before taking it apart or sending it off.
If the stickiness were due to a mechnical problem, then it would probably feel "Sticky" exactly the same way each time. And if this is the case, I don't think anything is going to help short of taking it apart or sending it off for repair.
It's a shame though- the action on the S90 is really quite awesome.