Camera can only take pictures sometimes?

I'm using a Nikon DSLR. I noticed that the Fxx/x thingy which i think is the aperture, becomes F --/- . There are usually digits after the F to show the amount of light but it becomes blank ( - ) sometimes and that is when my camera won't capture pictures. I can take many pictures at a time and the next second, I can't ( When the F becomes blank ) This happens at any light intensity.

All of your answers are in the manual.

That's your cameras buffer filling up, your DSLR has a processor inside that can only handle so many shots taken at the one time in burst mode or in quick succession, when your camera stops taking more shots it's because it's too busy writing the data of the shots already in the buffer, more expensive cameras have better processors and bigger buffers, it's just one of those things I'm afraid.

If it's not buffering, then it will be the lens not making good contact.
Clean the contacts in a senible manner and re-attach it.
Still no good. Try a different lens before contacting the repair people.

If you are in shutter priority or manual mode, and you vary the shutter speed to something the aperture can't handle, the F xx will disapper. Try shooting in Aperture priority and see if the problem resolves itself.