The F5 has a self-checking shutter. When it picks up shutter issues, the camera starts giving you the lockup and error message. Mine also does it occasionally, for one or two frames per roll on and off. It is at the point where I want to get it fixed, but I am not sure the cost at this point is justified yet. It is likely to be a shutter replacement, which is quick and easy and expensive (relative to the value of the camera). My D200 had a shutter replaced, and it was about $200, but the camera at that point was worth quite a bit more and I was depending on it for income, so I went for it.