Dave, I had the same problem about a year ago with the C above middle C. No one had an answer and I used an electronic cleaner all around B, C and C# to make sure with the cleaner. When I did this, I turned off the keyboard and after spraying, I pressed the keys rapidly to make sure the cleaner got in everywhere. I left the keyboard off for a good hour or two to make sure the cleaner had dried and I fixed the problem. Worth a try.

Mark