I have whole array of things that might have gotten bad, some of which can come and go, just as you wrote:
ripped curtains could make this pattern go on and off, check for loose thread sticking out of a curtain edge
badly serviced curtains (unevenly attached) - not very probable to manifest itself on a single roll only
Faulty development or fixing - it's a bizarre pattern, but you newer know... If you've done stand development, I would blame the processing part, not the shutter.
A hair or a thread somewhere inside the lens (somewhere near rear element?) - most probable cause IMO too, may vanish by itself
A light leak, as suggested, may manifest itself this way, or a faulty cassette
Finally, a batch of film might have been produced with a flaw like this
It's not really good to hear things like this to come and go, there's a chance of it coming back and ruining more shots. Prevention needs the cause to be known.