Tough diagnosis, MJ. I'm going to rely on instinct and a little experience here to take a stab at it. You've mentioned that you can play it some days, but not others. That indicates to me that you either have a technical difficulty not quite ingrained in your muscle memory yet, or some kind of stress/tension is creeping in. In some cases, it could be a combination of both.
Since you mention that the biggest trouble you have is between the arpeggios (you mention trying to make them "seamless", and that it works on some days but not on others--if I read it right, of course
), I'm going to start by suggesting two quick and easy things to check in your hand/arm. First, check your wrist height. If it drops too much, or at the wrong time, you won't be able to connect the notes. Second, if your wrist height is good, make sure your thumb is hitting the last note of the group, and then your full hand/arm weight is coming back with you to hit the top note of the next group. Often, when trying to play something fast, a common subconscious mistake is to lock the arm moving "down" the piano, and forget to let it go back and really hit the top note of the next group, so you hover over the keys instead of landing on the notes, and this really helps to create a "sometimes I hit it" problem.
If you want, think of it like football. The wide receiver has two jobs. First, he has to catch the ball. Then, he has to run with it. If he tries to run before he catches the ball, he will drop the ball. He has to be patient and catch the ball first, no matter how fast he wants to run. Same thing here. If you try to move your arm too fast to the left, your finger has to stretch back to hit the note by itself. This locks your wrist, creating quite a bit of tension, and kills any shot you have at consistency in the passage.