Just a quick update to the problem I'm having with banding. I e-mailed Dan Burkholder, who is a digital negative expert. He's written a book widely considered one of the best on the subject.
He had some good advice. The things that seemed most relevant to my situation were that the cheap transparency film I've been using could have aged poorly and the coating that holds the ink dried out, and that something could actually be wrong with the printer. To test both of these ideas, I took his suggestions and tried two things.
First, I cut a piece of the transparency film so that it could be fed into the printer rotated 90 degrees. By doing this, I could see if the transparency had dried in bands. When I made a print, there was still banding, but it didn't seem as pronounced as when the film is fed through the printer the usual way.
That didn't completely rule out a problem with the printer, so today I tried Dan Burkholder's other piece of advice, which was to find someone else with the same printer, and print the same file using the same transparency film in it. As luck would have it, the school I teach at has the same model printer in the Mac classroom. So, I went to school today (which was fun-I got to talk to lots of my colleagues whom I don’t see regularly due to my being on sabbatical) and made a print. If banding showed up on that one, then the likely cause of the problem would be the transparency film, and not my printer.
Looking at the print I made at school, I saw...banding! Nearly identical banding to what I’m getting at home, in fact. That means, the likely culprit is the transparency film, not the printer. I’m relieved, visions of having to buy a new printer have been dancing in my head the past couple of weeks.
So, now I have some much more expensive, but much higher quality transparency film being delivered tomorrow. When It arrives, I’ll try printing a negative and see what happens.