I was in the same boat once, I could handle slower pieces, make 'em juicy and enjoyable, but I was sloppy as soon as speed was needed. I knew it and I hated it. The solution was simple, I learned more speedy pieces and soon developed better control. That's a reason why I like learning easy classical sonatas like early Haydn - to learn the whole sonata you get to do both fast and slow.
Counterpoint is spot on, it's all about economy of motion.
I'd like to add that there comes a point where you have to stop thinking mechanically and entrust control over the details of movement to your subconscious. After you've figured out how to play something at speed, and drilled the movements, you should be able to forget about what your hands and fingers are doing and instead put your focus on the music... and to ALWAYS be listening carefully - the more picky your ear is, the more you will not accept sloppiness from yourself.
...It's a bit of a trap I sometimes fall into, that I keep thinking mechanically after I already know how to do it. Obvious as it seems, heh, I sometimes wind up fixing things that aren't broken (completely screwing myself up!).
I find this to be especially a problem with technical exercises like hanon, scales, arpeggios, etc. Because there isn't much music there, the focus is more succeptible to wind up overly mechanical. I don't think they're terrible or anything, but I find learning through actual music preferrable, because there's more to learn... When you have a passage of music you learn all the little nuances that give it character, and those nuances are technique as well. You can learn these things through hanon and scalework as well, but it's not so natural as in a piece of music.
Topic: How to improve technicals