Memtest gave no errors after 2 passes and the latest bios descriptions offer 'Modify AM3+ CPU voltage setting' and 'Improved UCC function' - could that help or not really?
Also if I had to get a new PSU, what affordable brand would you suggest?
So there is a newer bios? If it alters the cpu voltage settings that could be the culprit. Suggest you do a bios update to see if that solves the issue.