Playing video sometimes causes external monitor to freeze

<<. I've tried different cables but no change. >>

substituting a different HDMI cable is a check against BROKEN cables, but does nothing for inadequate cables, which is most of them.

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"Premium High Speed HDMI cable" or that + "with Ethernet" --OR--

"Ultra High Speed HDMI cable" or that + "48G"

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

HDMI was invented for HD TV sets. it works great at its original resolution of 720i or 720p. At higher resolutions, it quickly develops issues that are complex to solve, and the cables and adapters required to solve are NOT intuitive.

does you external display support copy-protected output using HDCP ?

if not, when you play a commercial move, that display will go dark.

Peripheral devices have known to have issues with Apple's Energy Saver. Use Screen Saver instead of Energy Saver to avoid such issues.