5. Your network isn't partitioned exactly, but someone bumped an ethernet cable and the packet loss has reduced the goodput on that link to a level too low to sustain the throughput you need. With most congestion control algorithms (basically, not-BBR), at 10Gb even 1% packet loss is devastating.

6. Well, basically the hundred other reasons the machine could brown-out enough that things start timing out even though it's sporadically online. Bad drive, rogue process, loose heatsink, etc.

Dead hosts are easy. Half-dead hosts suck.