This has been quite an odyssey; I first had a velocity problem last fall (2022) with the D above middle C, and went in and did the "clean and swap" as described earlier in this thread (such a helpful post by intuit!). I cleaned and swapped the rubber contact that included the D with the one at the very top; the D4 worked fine (for a while), but the one I'd switched still had the velocity problem - it happened to be the highest D. But I considered it a success since I rarely play up there.
Then I started having the problems I mentioned in my previous post: The middle C was getting stuck, and the D above it had started to have the same velocity problem as before. I thought it might be good to try the solution a couple others have - getting a whole new PCB to replace the current one.
So I open it up and find the craziest thing with the middle C - it was getting stuck in the felt pad! Somehow the metal part of the action had opened a gap in the felt - I'll try to post a pic. As a stop-gap (literally), I cut a piece off the end of the strip (it goes the whole length of the keyboard) and glued it in, and that seemed to do it.
Then I replaced the PCB with the new one, and it's working beautifully. The tone and touch seem to be much more consistent with this new board - the original actually had different touch between the black and white keys (I thought I was making that up until I tried it with the new board and heard and felt the difference).
Short story long, if you're having any issues at all with velocity or unevenness in tone, I highly recommend getting the new board. Kawai apparently says it's a significant improvement over the first one, and I can attest to that. You can find it here:
https://www.synth-parts.com/en/products ... -044-2?c=0
I'm far from a mechanical genius, so I can attest it's very doable, especially with the pics from intuit's post. I'd be happy to help anyone through it - I did take some pics.
Of course, there's the issue of why a $4k keyboard should have any of these problems; obviously it shouldn't. But imo there's nothing else like it - action, sound, looks - so I'll stick with it and hope this solves things.