Mac Mini M1, 16GB - Video Playback Issues

Mac Mini M1 16GB, 1 TB, everything up-to-date. Two monitors attached to the box, a third through a DisplayLink compatible hub.

I have an issue where I'm reviewing a load of MP4 video files in VLC (or IINA or simply Preview) and after an indeterminate, random number, they simply refuse to play. They won't play after that in Preview, or QuickTime or anything else. The first frame loads, but no play.

They also then don't play in a browser , i.e. YouTube, etc. I've also noticed today that this is accompanied by unresponsive audio volume keys whereby you change the system volume, and it takes a minute or so to react and show that volume change on screen. This leads me to believe that some part of the media system is being dragged down to really unresponsive levels, but no idea what.

This has happened a few times in the past few days since I got the machine and each time I have ended up having to reboot.

Any ideas? Many thanks in advance.