If its time related then start investigating power related things. The PSU would be the first thing to look at. Try powering only the bad blade pair (as long as they're the ones that power the controller) only and see if they still drop out. Alternatively, you could just swap your PSUs over and see if the problem moves.
Sometimes it happens after five minutes, sometimes it is overnight, sometimes on boot. I'll give swapping the power supplies a try next...
Swapping the power supplies had no effect. I guess I'll see what Bitmain has to say (they got back to me pretty quickly and asked for screenshots, etc), which I've just sent them.