Have you tried a diferent card ? If you are getting the same problems with two or more different cards then I'd be very concerned and take it in for servicing. I've had only one problem with my D70 where I had an error message regarding my card. I re-formatted the card and it's been fine since. Flash memory cards need to be re-formatted periodically and I apparewntly neglected to do so and the camera let me know it.
Great photographs are never taken they are created.