Rear cassette shifting
You have an SRAM X4 8-Speed shifter. The description says: 


  Featuring our proven ergonomics and award-winning 1:1® Actuation


and adverts in e-shops say:


  Compatible with ESP (1:1 ratio) rear derailleurs only.


The pull ratio for Shimano rear derailleurs is 1.7. It is not compatible. You need a different shifter or a different rear derailleur (like the X4).