It can't be stress marks. The negs are intact looking (no dents, dings, etc.) and the problem occurs with two seperate cameras. O.k. Both are Nikons but they are almost 30 years apart in age. So it can't be that.
Although the point about visible discharge is worth noting here. We are talking about film rated at 6400. The same shots with 400 or 100 speed would be pitch black. This could be a regular occurance, but only a problem when I am running such funky film through the cameras.