fast passage---EXACTLY how to practice?

[piani0player]

hi everyone
   After spending a lot of time practising all this fast  passage in the Mozarts sonata ,Bachs prelude and fuge and the apperggios passage at the end of the Chopin etudes op25 no.1. i still struggle to play them up to speed evenly and equal in tone quality.could someone just tell me EXACTLY how they practice fast passage??
i think i still dont know how to practice fast passage.

     This is how i practice fast passage----play the passage with the m.m. at a comfortable tempo with different rhythm than gradually increase the speed and
keep using the m.m. until its up to speed,sometime i can play them well but the next day it all become uneven and can't play up to speed.what wrong?

     For the chopin etude op25 no.1 , i still can't play the last apperggios run up to m.m. =104 per beat . i can only play it at about m.m. = 60-70 per beat?how to practise them correctly?
 thanks for your help