Before I received your message I happened on to a similar setting on my D700 which may explain why I was having shutter release problems. I was setting up my D700 to shoot a rugby game today and noticed that the focus-mode selector on the front of the camera was set to the S position. I don't recall changing this switch after the volleyball games, so it appears likely that I shot the volleyball action with this switch set to S rather than to C where it should have been set to provide continuous focusing for moving subjects. Ugh! I always have the AF-S priority selection set to "Focus" rather than to "Release." This serves to explain why the shutter would sometimes not release when I was photographing the volleyball action. After shooting numerous sports events for the last four years I can't explain how I could overlook setting the focus-mode selector to C. Some things defy explanation! Now I know one of the things to definitely check if I ever encounter shutter release problems in the future!
In shooting the rugby game today I didn't have any shutter release issues with the focus-mode selector set to C. Problem solved--I hope!