Looping through the same lines while solving puzzles When solving tactics sometimes I spend a lot of time (20-30 minutes) looking at the same line over and over again. 
Sometimes I have a feeling that one particular line is the right one but my vision gets blurry after 4-5 moves and I'm unable to go on without moving the piece.

Do you have any suggestions (books and/or training methods) on how to improve in this area?
Establish an order to analyse the lines!

I would begin by the most forced moves (checks, mate threats, captures...) and finish with those where the opponent was multiple possibilities. Respect that order strictly so you won't be jumping back and forth.

Use the same criteria also for sorting subvariations (ie: start by the most obvious check, pick an opponent's answer, now calculate checks, threats and captures after that particular answer and so on)