I don't know what Nikon calls it but set your camera to use "expanded central spot". That uses about 9 focus points near the center of the viewfinder (or wherever you move them) to focus. Single point to too hard to keep on the subject and, as you have found out, using full frame can confuse the camera. Manual on infinity is not the answer as depth of field is very limited with a long lens. A 400 MM lens, at f/10 on your D800E at 60 feet has only about 5" of DOF! Also, check to see if your lens has a focus limit switch. You may be able to help the lens if it doesn't have to run through the entire focus range from minimum focus distance to infinity.....something like 30' to infinity....