Yeah, I've noticed that with most long running series...I begin to lose interest in the characters after a certain point, particularly the episodic series. Although that happened with ER, I gave up sometime in the 7th season, then came back for the 11th or last season.

Some series get better as they go, like fine wine, but a lot of series I've realized have a shelf life of 5 years. Go past 5 years...and it begins to wear thin.