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Ah, your response actually provides a couple clues that might be relevant. Slow practice is certainly useful – and essential in some situations – but it has some inherent drawbacks too.
Because usually, when we do slow practice, we don’t play slowly the same way we do when we play fast. In other words, we might use more bow, allow the fingers on our left hand to lift higher above the string than we would at fast tempos, etc. So then, when we play the passage at tempo, we’re suddenly having to play it differently than we’ve practiced doing it.
So this might be a situation in which slow practice is actually reinforcing some “bad” habits, or at least motor movements that you aren’t able to use at tempo. It might actually be more helpful to practice these passages at tempo or even a bit above the ideal tempo, so you can practice releasing finger tension, and playing with more ease. At first, this is probably going to be a mess – because the primary goal may be to focus on minimizing tension and playing with more speed and ease as opposed to worrying about accuracy and getting all the notes correct. And you may have to practice this with a metronome in very small chunks. Like building it up by adding one or two notes at a time until it seems to stabilize. Because at the moment, it sounds like making the passage easier for your left hand may be more of a priority than making it more accurate at the expense of speed and physical ease. Make sense?
Noa
