Familiar problem with a stutter

Hi all, been seeing many versions of this issue which seems to still be an enduring mystery: why is playback/recording playback suddenly stuttering?

It’s made work damn near impossile during the past week and I can’t understand what is suddenly different.

I’ve tried updating to 3.7.1, cutting out background processes on Task Manager, and fiddled with sample rates on my projects but after the first 45 seconds its right back into the same thing… Torture because I have an album to finish!

EDIT: only seems to be a problem with larger multi-track projects, when I drop a completed song file back into audacity it goes without a hitch