Thanks everyone for your help, it’s much appreciated.
I’ve tried now running large matrix multiplications in a loop on both CPU and GPU. This leaves nvidia-smi showing ~250W out of 250W power draw on the GPU while htop shows  ~100% utilization on all 24 CPU cores. I ran this for 1.5 hours, with no crash.
I also tried throttling GPU power via nvidia-smi to only use 200W, and then running the PyTorch neuralnet code that has crashed before (num_workers=8). This crashed after a couple of hours, which means it’s ran longer than it usually does. But I don’t know if this is because of lower power draw, slower execution, or if its just a fluke.
This leaves me somewhat more confident that it isn’t the PSU. Do you agree? Are there other tests I could do to exercise the PSU?