So update on my PC that used to have system crashes ALL THE TIME in Linux - I think the new GPU has solved the problem. Not a single crash in the last few weeks of reinstalling 18.04, after changing the GPU.
However, my Wi-Fi card is still having issues. I have to disable and enable wireless like every 10 minutes. New card getting delivered today. Hopefully that's the end of that.
Finally, I have an old GPU if anyone wants it? Pay postage and it's yours. It's a 2GB GT 610 by Asus.