What your describing sounds allot like a power issue for the RasPi. Especially the problems starting after loads of writes (pacman -Syu).
I used to get loads of corrupted SD cards and weird issues when I had my PI running of the same hub as the Erupters.
After moving to a dedicated 1A power suply the Pi has been stable.
I no is sounds exactly like those power issues, however the PI is the only kit with a dedicated power supply it runs fine through all the writes..
It was over an hour running before I thought hey I should give it a reboot.. then bam...put the image on a different card repeat.
I think I wrote the image to 3 cards about twice each yesterday

I'll get to the bottom of it but I really need a second PI to screw with..