If anything, I would zoom on “disk was not cleanly unmounted.” To me that indicates that either the disk disconnected by itself (or that card related), or system just shut down. In both cases, to me that would indicate that you may be out of power, or at least, this is where I would start.
Unless you are plotting, shutdown or disk disconnect should not cause problems on fsck level (I think), as there are no writes going on. This is why your fsck comes back clean (again, I think). If on the other hand, your plots are mangled, that would rather point to SATA controller. (Again, if you are not actively plotting, as only those files could be mangled, I think.)
You have mentioned that you have 5 video cards. Assuming that one draws 200W, that puts you at 1,400W right away. I am not sure, whether those video cards also take 5V, maybe not. You add to that 10W / drive, plus some extra for CPU / RAM / mb, and you are pushing close to raw 1,600 limit your PSU has.
Instead of dropping down the number of drives, I would drop one/two video cards, as that is the biggest power draw, and let it run for some time.
You may go through your /var/log/messages to check what were the last lines before the system restarted. If there is nothing there indicating problems, but some noise during the startup, that would further suggest that the system shut down abruptly, further suggesting power issues.
One more thing you can consider is adding a small UPS, as that would eliminate potential small brownouts coming from the power line. Maybe your fridge kicks in, and the power drops enough for your PC to shut down.