With the second passage, you may be trying to use too much energy from your fingers instead of just dropping them down with your wrist. My teacher has done this piece before, and is teaching it to me now. Try this:
1.) 1-4, your hand should be positioned like you're playing an octave with those fingers. Point your wrist to the right.
2.) 2-5, using your wrist, lean into each key so you're not putting any strain on your hand.
3.) 1-4, bring the wrist back to starting position
4.) 2-5, leaning in once again with your wrist.
The easiest way to practice this, is to repeat the first four groups of sixths in the piece continuously in
p then
f dynamic. Since this is your first piece that involves sixths, you may want to practice the first two sixths as a trill, doing only those two notes for the duration of the bar just to get warmed up for what's ahead.
The rest becomes basic note reading. With this method, you won't have to spend as much time building up the strength you need to play the passage fast because your using less energy in your forearm, where all your finger muscles are. The most important thing, however is that you don't attempt any of the 6ths with fatigued fingers.
Good luck!
Topic: Problems with my Sixths, thirds etc. technique