Playback glitches when clicking other monitor

Using Cubase Artist 8.5 on a MacBook Pro 2009 laptop with a 20" Viewsonic external monitor. OS X El Capitan 10.11.1 Intell Core Duo

Runs great except when I’m playing back and click on the other monitor, it glitches (mutes) for half a second. Then when I click on any window of the first monitor like the transport, or editor, or a plugin it glitches again. It doesn’t do it if I keep clicking on windows in the monitor that’s in focus, but once I try clicking on any window on the other monitor it will glitch. Any ideas what’s going on?

No one has this problem with an additional monitor? Clicking on the other screen makes the sound glitch?