[matthew-wegner]

> that changes randomly

Moving the mouse pointer to middle of bottom edge of a screen will permanently move the Dock there. As far as I know, there is no way to disable this behavior. It's terrible.

For simultaneous multi-monitor, you're probably forced into using a full Dock replacement. There are a few options out there, but none have ever stuck with me personally.

[chedabob]

If you go into System Preferences and disable "Displays have separate Spaces" it'll keep the dock and app switcher on your primary display.

I'm not a Spaces user so not sure what impact this has outside of the dock behaviour.