If I were going to start out I would unplug any unnecessary (to operating the PC) peripheral. Then boot up inclean boot mode
(much more comprehensive than the safe mode I was going to recommend) and then shut down.
This rules out peripherals and their drivers as well as other non-essential drivers and services, any apps in the startup folder as as a cause if it still shuts down abnormally. Then check BIOS update, chipset drivers, other main drivers for updates.
If a normal shutdown then start adding stuff back in chunks until you find a group that causes the issue, then narrow it down to the culprit from there.
Try the troubleshooting tools built into the OS (type troubleshooting into the run box). Reliability Monitor is unlikely to be of benefit since this problem started at installation.
I forgot: Was this a clean install of 8.1 or an upgrade from Win7?
Normally I advocate making sure the latest BIOS revision is installed before an update but the last one available was pre-Win8 and the fixes don't seem relevant to me for this situation.
[hattip to the Admin on Windows Secrets
for link and idea.]