I had the same problem with two of my cameras that were a little farther away from my NVR. I finally solved it by putting a wifi repeater halfway between the cameras and my NVR. That solved my problem. They are much more reliable now.
I have thought of possibly doing the same.. The only way I could think of doing it would be with some sort of wireless HDMI device. Not sure how it would work, been meaning to give it a try, but haven't gotten around to it yet.