04/03/1995 at 01:00 AM EDT
TV SHOWS RARELY IMPROVE OVER time. The most you can hope for is that they manage to sustain quality from season to season. Eventually, key creative talent gets bored or moves on, the premise and/or the characters are exhausted, or the series simply loses the chemistry that made it special. This season two fine shows, Northern Exposure and Murphy Brown, have passed into the valley of fatigue. Don't be fooled by the perky episode a few weeks ago that featured Pee-wee Herman as Murphy's wheedling secretary. After seven years this sitcom has run out of steam, while Northern Exposure has lost touch with the quirky, quasimystical tone that gave the show its appeal. CBS will soon announce its renewals for next season. Murphy Brown will likely be back; Northern Exposure is on the bubble. Either way, their best nights are behind them.