hi. i guess your problem matches mine with a 7 body. i bought it a few months ago and shot just one roll to test it, it went just fine. now on a random basis (let's say 3 out of 10 exposures) i advance the film, shutter won't fire, advance the film again, shutter usually fires. By closely listening i managed to hear like a "click" when releasing the film advance lever after advancing the film. when it happens, the shutter won't fire. when it does'nt, the shutter fires. placing a finger underneath the camera on the curtain rotary knob yuo can "feel" this click as a light vibration. From what I gathered by browsing the internet, it's related to a magnetic switch which tells the system that the shutter has fired. now, I think this switch should be operated by the firing button (in one way) and by the film advance lever (in the other way). for some reason i think sometimes the film advance lever "charges" the system on its way up and erroneusly "discharges" the system on its way back.