Bought a 645AF a few months ago, and it came with two backs. Within a couple of days of using it, I noticed one of the backs would reset to '1' part way through a roll of film, making it impossible to know how many shots I really had left. It would also do the 'leader wind' action when I next pressed the shutter actually it's probably the reverse order of those steps, but I wouldn't notice it until the back made the extra-long wind noise. Anyway, I replaced the battery in the back and the problem went away. It happened again today. I have no idea if it's the same back or the other one, as I didn't mark it (thought the problem was solved). There's no low-battery indication on the back. Is this a sign of low batteries? Is the back door coming loose momentarily, causing the back to think it's loaded with fresh film? Something else? Basically, is this a known problem/feature or do I have a weird back? Thanks.