If lack of strength is the problem, try practising the offending passage leaving the third out, so that you only play the melody note 'A', for instance, instead of both the 'A' and the 'F#'.
Then play the melody notes as loudly as you can through finger action - won't sound pleasant but will help to strengthen your finger.
When you play the passage as written, try leading with your wrist. This evens up the length of your 5th and 3rd fingers a bit and means you will be less likely to split the thirds; also your weight will be travelling towards the note that needs bringing out.
Try going up the passage playing each beat 6 times, then 5, the 4 etc. This gives you more time to work on the strength without having to worry so much about hitting the correct notes with each move.
To relieve the pressure of playing the passage, indulge in a bit of rubato - a rit from about half-way through sounds great and helps technically
Hope this helps. Enjoy the piece - nearly everybody who plays it loves it.
Steve