Steam download speed keeps dropping to zero

Trying to download a game on Steam right now, but the download speed is all over the place. My internet is fast—should be getting well over 50MB/s—but Steam keeps dropping to zero and slowly creeping back up. It's been doing this on and off for hours and I can't figure out why.

I've already checked my connection and it's fine on other devices. Anyone know what might be causing this or how to fix it?