What happened with the system crash? Did you reformat the hard drive and install El Capitan as a fresh install? Have you tried resetting the PRAM?
The update that was discussed seems to affect macs at random. I thought it only affected those with a dedicated ethernet port, but I've seen some say their adaptors were affected.
I've used the ethernet dongle with thunderbolt for the past year and a half and it's always been decent, but not nearly as stable as a dedicated port. I would try the booting resets (PRAM AND SRAM), try a fresh install, and if none of those work I would contact apple and work through if the issue is the computer or the adaptor.
