Has a manga series ever affected Jump or similar magazines sales a lot? For example when Naruto started or ended has the sales ever fluctuate?
I've found you an article that show some graphs about Jump sales, and data about their best selling mangas that I think you'll find useful :

source

And some date from best selling mangas to help you evaluate those graphs : 

Saint Seiya (1986 - 1990) 

Dragon Quest: Dai no Daibouken (1989 - 1996) 

Hokuto no Ken (1983 - 1988) 

Slam Dunk (1990 - 1996) 

YuYu Hakusho (1990 - 1994)