I had a similar issue with an r730xd. Spent ages troubleshooting and replacing PSUs. Ended up being a fan I had plugged into the internal USB to cool a GPU. Removed that and never had any power issues since. Just incase you had done something similar
homelab
Do the PSUs have lights on them? Do those lights turn off? How is the power quality, clean or noisy? Are you using two power leads, or a Y-cable? If two leads, to the same circuit or separate?
I would agree that it's probably the backplane, and you can probably find one on eBay or Kijiji for relatively cheap.
And of course, if you haven't already, check the idrac event logs for anything interesting, and try the diagnostics.
The PSUs have lights on in the normal manner, they both have green lights when the machine is unresponsive. The power is probably OK, it works well enough for many other systems. The PSUs are configured in a redundant configuration, I've tried it running each of the PSUs separately and running both. Tried running both from a Y cable and from separate cables. I've tried switching the PSUs between bays. None of this changes it. But all on the same circuit.
There's nothing in the iDRAC logs related to it or at the same times.
Thanks for the suggestions. I'll try it on a different circuit but I have to put it back together again.
Yeah if you've eliminated all of those, it's probably the board.