[teki4u]

My Asus Chromebox has recently started behaving very oddly when playing videos

Purple lines flash across the screen

Green-tinged frames JUMP in and out

Sometimes it just goes black for a second or two.

All in all it's making it pretty unusable

Has anyone had similar problems? I can post a debug log later when I get home.

I have a feeling it's a hardware issue as it has worked fine for a year or more

I have it too and, i was told it was a handshake issue same as you describe and i also posted photo. I've tried everything possible changing OS (currently running windows 10 off external HDD), downgrading coreboot firmware but nothing fixed it so yeah I guess is a hardware issue.
Though I've been able to make it behave a bit better by pushing the box against the wall with HDMI cable making some pressure in straight position, now I get occasional 2 to 6 or so seconds of "black screen" with that little change, no bad coloring behavior anymore.