I picked up a used 645AFD at an auction some weeks ago. After some days use it suddenly starts to lose contact with the film back. When it happens the AF also turns dead. Normally all that takes to fix it is to insert the dark slide, detach the back and re-attach it. But then the back can get a bit confused and start to wind the film (like when you've loaded it with a fresh roll). I've changed batteries of course. I've looked at the connections between the back and the body and they look nice. This really pisses me off I must say. Anyone experienced this?