When I mean tailed off, I mean movies that kind of lagged and kind of gotten boring like in the 2nd half of the movie even if you love the movie. I will give a few examples, I love these movies, but towards the end, they kind of strayed away from what they were doing.

Stripes (1981)
Popeye (1980)
Smokey and the Bandit (1977)
Let's Do It Again (1975)
The Substitute (1996)
Nothing To Lose (1997)

That is a few I named. I will think of more later.