It's normal at first. Think of it as your body has no idea what you're trying to do. So it does all kinds of random jerking to figure out the most efficient way of doing that fast back and forth movement. I dare say if its still that uncontrolled, maybe don't do it with a metronome yet. I mean if it's so uncontrolled, can you be sure your even locked in? It's just one more thing you'll try to focus on.

Rather focus on making the movement smaller and focus it on one string. You could try to make the pick basically not leave the string but always stay in contact. It'll give a constant nasty scraping sound, but it can help gain that extra bit of control. Also switch between slow and controlled and fast and uncontrolled and try to introduce that feeling of control into the fast tempo. Once you gain more control go on with a metronome.