Based on the information you have provided so far, the shutter seems to be okay: photos are being taken without a problem, even if the viewfinder "goes dark".
The problem appears to be with the mirror, which is responsible for redirecting the light from the lens into the viewfinder, thus giving you a picture there. Normally, it swings up whenever you press the shutter release to allow light to fall onto the sensor. It seems like it is "hanging" occasionally and does not swing back into its original placenormal position, thus the "missing" image in the viewfinder.
You may want to take the lens off your camera and take a few shots until the viewfinder problem occurs again, then look at itinto the camera; if the sensor is visible, then the mirror problem is confirmed. Maybe you can also see what's obstructingkeeping the mirror and keeps it from returning intoto its original position.
The rest is up to you: if you feel comfortable around taking cameras apart, feel free to grab a screwdriver and get to work! ;) Otherwise, you may want to seek a certified Nikon repair shop or a free one. Either way: Best of luck with getting it back to work!