Prelude op. 23 no. 5

[l3g0br1ck]

Hello everyone,
I've been learning Rach's prelude op. 23 no. 5 for about 2 months now (I don't have a lot of time to practice), and I can very well play everything very well. But I still have one main problem: I can't play the three repeated chords together very quickly and clearly, it's ether slow or very unclear and often results in two ugly chords    I'ts quite a problem when it's the characteristic feature of this piece 
I would like just to know if there is a particular technique to learn, a movement I should do to make these chords or just practice, again and again ?
(I really don't think it is a question of level, I've been playing for 11 years, and played recently Brahms' rhapsodies, Schubert's Impromptu no 2, an etude of Chopin and Rach's prelude in C minor too, I think this is of my level and I don't struggle with the bigs jumps and things)
I hope you can help me !
Thanks