I have had the same problem with a brand new s7 392 i7 processor.
I spent a week applying windows updates to the machine and though it may have been someting I installed.
I reverted back to factory installed settings and same thing happened.
It would shut down randomly at different times from when it started to book, to the windows signon screen and during working. It didnt matter whether it was on ac power or battery.
I was told to send it to acer for repairs and told that I may have recycled parts used to fix it (This is a new machine $A2200)
It was tested and acer told me there is nothing wrong with the machine during their testing
I told them that their testing cannot be adequate enough as 1 I have experienced no end of frustration with this problem and 2 had plenty of witnesses to prove it was happening
I bought this machine to support my place of work and it has cost me big $'s in lost productivity.
Acer do not seem to be interested in the fact that this appears to be a common problem with their machine and just fob you off with inadequate responses.
I insisted on a complete refund on the machine from where I bought it and under Australian consumer law section 52 they had to comply.
Would never consider buying an acer again.