One thing I had forgotten about. When I experienced the focus drifting off, the focus initially locks on, but as I press home the shutter button, it loses it. Is that how it looks to you? If so, then what's happening is that as you press the shutter home, the camera then tries to refocus again and in doing so, it loses the focus it initially had. One way around this is to set up a back button focus on one of your function buttons. When you do this and press the back button, the camera locks focus and pushing the shutter button more won't have any further affect on the focus, but you must disable focus from the shutter button. In other words, once you disable the shutter for focus, all it will do is take the picture. All focus will be done from the back button. I haven't set mine up yet, but will do so soon. I have it on my FZ1000 and quite like it.
