Mendelssohn Variations Seriouses

[clara.schumann]

Hello,
i am currently playing the piece: variations seriouses by Felix Mendelssohn.
if anyone here ever played this piece i would be glad if he'd help me- in the 13th variation the theme is in the left hand and the right hand should be like "floating" (i really hope thats the correct word  ) and quite with a stacatto.
it turned out to be very hard to do a fast and wuiet stacatto, and many times i just dont succeed in pushing some of the notes. how am i supposed to work on it? the problem is mostly with the 3-4-5 fingers so what should i do to make it work in a normal tempo?
thanks in advance