To my knowledge, when an OS such as Windows 8.1 is booted, all firmware/bios drivers are replaced by more appropriate OS drivers in the boot drive during start up. I learnt this happens for the latest versions of Windows. Am I right? Do all bios drivers get replaced? Does this happen with the latest versions of OS X as well? If this is so, then you can discard firmware as suspected of causing problems on a computer that's already booted up. Unless there's a problem that's causing a driver to remain in RAM after boot.