Yamaha P80 Sticky Sluggish Keys
This is an answer rather than a question. After putting up with 4 sluggish sometimes stuck keys on my P80 for 3 years I finally took it apart. There were eight keys with little fractured off pieces at the pivot point that I assumed were jamming or rubbing in some way so with them removed I thought I would be good to go and was for about one minute. Apart again I deduced that it was simply too much white grease on the little rubber contact point and the weighted mechanism. Just needed to wipe the pad but I can see that perhaps in some cases you would need to wipe reassemble, take out and wipe again if there is waaaaay too much grease instead of just too much. I believe it is sort of a suction sort of thing that prevents free movement.
This is not a job for anyone who is not a tinkerer. Once the cover is off (ton of screws but only the large ones)examine the very last bass key to get an idea of how it works before you start. There is a tang that will release the pivot point of the key when you slide a thin kitchen knife between the keys, push down ever so slightly, feeling for a springy resistance rather than solid stop feel, as you pull up on the pivot end of the key. Solid stop feel means you're on the wrong side of the key. Tang is only on one of the sides.
Removing the key entirely is a bit dodgey. It feels like you're going to break it and reassymbling is even harder but it is pivot point first. Then holding down the actual weight, once you've determined which one wants to lift, pushing the exposed end of the key down hard enought to pass the connection point. You can possibly knock the little rubber pad in this process and that's going to mean taking the bugger back out.
Again if you're not what's commonly referred to as "handy" I wouldn't recommend undertaking this.
My dictum is: "Anything can be fixed if you fool with it long enough."