I have the GF670 and I have also had this issue on occasion, but with all sorts of films - Acros, 400H, Portra 400. Sometimes the roll doesn't wind tightly enough, and at the end of the roll it can get really hard to wind on (like you, I have been scared of breaking it). When I take the roll out, the film is so "fat" it bulges out of the spool, causing small light leaks at the edges of the film. My theory was that it's caused by rapid change in temperature.