Does this dark-light-dark-light pattern happen if you reprint the same exact film several times? Or are you saying that it happens with some art (or layers within a file), but not with others?
If you can print the same thing twice without changing settings and the output is inconsistent, it sounds like a printer issue. If certain art or art layers will print dark, and other art or art layers print light, and this remains consistent, I'd guess that you're not always using the same value for black. (As a related possibility, I might look at whether the variation is dependent on the color mode of the file, i.e., RGB vs CMYK.)
Another thing to check is whether your print settings are remaining 100% consistent. A change in settings can often have a drastic effect on output.