I have a problem with a Spotmatic II I aquired recently (mainly for the lens). When I wind on, the shutter will often fire as soon as the camera reaches full cock. If I wind on by inching forward or do it very gently, the camera will work perfectly.

 I have just worked om a Spotmatic SP which is similar to your camera. I am afraid your problem may require one more level of opening up the camera. After removing the front face cover, you will find a mirror to shutter release cam at the top near the m