A recently purchased ($12 @ KEH) EOS ElanII has a bit of a problem. Every once in a while, the shutter will fire, but the film won't advance. A second press of the shutter will then advance the film. Upon developing the film, I find that these misfires result in either blank or dramatically underexposed negs. This happens very randomly. The camera will make 12 good exposures, then the next two will misfire, which leads me to think it may have something to do with the contacts on the lens. Any ideas?