TV series usually don’t change much over time. Good ones stay good and the bad are irretrievable. Yet some shows do fluctuate. This season, for instance, Davis Rules, Designing Women, The Fanelli Boys, Wings and Babes have all been sliding downhill. Law and Order and Seinfeld have consistently exceeded expectations. I’m also looking forward to the return next week of CBS’s Northern Exposure; its loopy humor held up very well over last summer’s trial run.