I have an FM2n that I got on eBay that's in mint condition - except that the film advance lever will advance the film whether or not the shutter has been fired. If I dry-fire the camera with the back open and then work the lever, it will turn the spool, advance the counter and reset the shutter normally. Then I can work the lever again. At the beginning I feel something catch a bit then give easily. For the rest of the action it has a bit less resistance, the spool turns and the counter advances, and at the end of the action the shutter gets nudged a bit. Can anybody tell me what's going on here and what to do about it?