I've experienced this same pair of errors with Corsair Dominator 3200 on a Maximus VIII Hero scgt1, also with a 6700K. As per your situation they occur randomly and unpredictably - and no amount of tweaking in the UEFI has so-far cured it (this tweaking includes raising the SA and VCCIO voltages unfortunately). The memory runs rock stable (and overclocks significantly) once in Windows and doesn't cause any problems on another board.
One workaround I found that brought reliable booting (at the expense of performance) was to drop the RAM speed to 2400MHz. The only fix I have found for this that maintains RAM performance is to downgrade to the 2202 UEFI. I then get 100% reliable booting - with full memory perf - but lose out on disk subsystem performance thanks to the older Intel RAID ROM revision. Not ideal either...
I spent some time comparing and contrasting the subsettings defined automatically by the 2202 and 3401 UEFIs to see if I could find some settings in the former that, if manually set in the latter, would give reliable booting. While I found a couple of minor timing changes, setting these manually in 3401 to the values set automatically in the older UEFI improved, but didn't solve, the POST unreliability. I reported this to ASUS about three months ago but haven't received a reply. Any chance you could try downgrading your UEFI to 2202 and seeing if that solves your problem? If it does I'll send another support request to ASUS .
