One of my teachers (I have two, technically; I study primarily with one, both he and his wife studied at the Guild Hall in the UK and now teach in Portland, OR, so I get tips from both) also taught me to always keep my arms moving (play your arpeggios with your arms, she might say). Practicing this slowly is essential, but you don't need to waste time working it up with the metronome once you have it done. Moving the arms as a unit really worked for me, don't know if it'll work for you or not, but definitely worth a try.