How about The League Of Gentlemen? Series 1 & 2 were superb, probably the best sitcoms of the past 20 years, but by Series 3 they'd decided to get a bit clever and each episode was set in an identical timeframe, focussing on one main character leading up to a car crash. It had its good bits but was nowhere near as good as what had gone before, almost lapsing into unintentional self-parody at times.