I do still have another PC handing around, but I’m not at the point where I’m going to reboot that yet. I see a regular supply of others on this forum with the exact same issue. Would be a bit of a coincidence if we all had the same hardware issue.
This PC is much better than the last one in terms of everything. So I doubt its a resource running out. Obviously I don’t have a HDD 
Currently on 5.5, it’s been a problem with all versions of 5.5.
Path lengths are all the same, its the same project.
Thanks for casting your mind over it, though 
I did the sacred ‘bin the derived data cache’ move yesterday. Everything’s refreshed now, and it hasn’t happened so far. I’m only moderately optimistic though 
Some people discussing in 2024
I think this rules out hardware