This is hard. She probably feels little control over her behavior, like a record with a scratch (remember records? oh well)---But, the thing to do is to tell her what she is doing, and to ask if there are ways she can change the subject, or allow you to "cue" her when she reverts back so she has a chance to change course. It will hurt her feelings, but I think she can feel what she is doing, and probably isn't any happier about it than those who listen to her. (I have some experience with getting stuck & repetitious)
Not sure what you mean by perverted, but that, I might just object to in the moment, let her know it is not a topic of interest....
Good Luck.