D800 I like it and then I hate it.....

Perhaps I've missed in this thread, but it would be important to be really clear about the OP's AF settings.

AF "Mode" is AF-S or AF-C set with the rear dial. Either should work.

AF "Area Mode" is Single-Point, Dynamic-Area (9, 21 or 51 pt), 3-D Tracking, and Auto-Area, all set with the front dial. I would always want Single-Point for full control. The worst is Auto-Area where the camera trys all kinds of things and is totally inconsistant. If the OP used this, it might explain his problem.