I do not have a spare PSU. I will soon request an RMA for it, but I doubt the issue will be reproducible so I'll just get it right back (as happened with the mobo). Likely an unholy alliance of components and drivers that has doomed me to keep a £2000 junk heap.
If you can fix it by messing around with the software (I think you said changing literally any option solves the problem?), then I don't think there's a hardware issue here, but I also can't say what the problem is likely to be. The people who made the software might have a better idea.