Bill’s suggestion is most often right.

If that doesn’t work, those symptoms indicate poor or erratic contacts of the key mechanism. Often, just hitting the key multiple times can wipe the contacts and create more consistent contact. The fact that a single “C” note is louder than the others indicates that the velocity sensing process is not working right. You can check the velocity sensor by setting the Overall Touch Sensitivity to zero (See the top of page 179 of your owners manual). If the key is the same volume as the others after that, velocity sensing is not working properly.