For the ones taken with the Pentax, the flare is consistent from one negative frame to the next. It is in the same position, and is a clear and sharp X.
It could be a problem of the camera itself. I was using an old Yashica with an extension tube for pinholes, and always had the same flare in all images. Same place, same pattern. After some trial and error it turned out to be a light leak from the view finder window, which was solved by using the plastic cap that slots over the view finder, provided to prevent light entering the view finder, which was no longer light tight.
The flare in your image from the 4x5 is different, blurry and amorphous. I don't know what to suggest for that one.