I've started having this intermittent issue on my Nikon Df.
Occasionally (1 out of 50 shots?) I take a photo but the viewfinder goes dark and doesn't re-open.
When i press it again it re-opens. The photo still captured perefectly.
initially i thought this was a lens issue but it's happening with different lenses so likely the dslr.
does anyone know what it is exactly and best place to fix it (refurbished out of warranty)?
From the explanation of what your issues are, it sounds like you have mirror lockup enabled. It causes the first push to lock up the mirror then the second push to only operate the shutter then release the mirror. The purpose of this is that the mirror flipping up can cause some images to loose quality from the camera shake and its more obvious in longer exposures where your trying to use a tripod to get tack sharp photos.
I would refer to your owners manual to where that is set in your menus of your camera.