I do this too but with music! It doesn't happen to me now that I'm on meds but before I was diagnosed and really manic, I would get one line from a song stuck in my head and it would just repeat and repeat for days, driving me crazy! I don't think it was the typical way music gets stuck in people's heads. It would just be the one line over and over again and it wouldn't stop. Is there an obsessive side to bipolar? My brother has OCD, maybe I have a little bit of it, too.