I've been encountering a similar problem with a Windows 8.1, homebuild using Asus Z87-K, i5-4670 and the Corsair CX600 PSU I used as a base for a homebuild at the weekend.
No BSODs, but keeps restarting at random. Could be several times immediately following login, could be 5 mins into browsing online. Very frustrating!
Having researched a lot online I started to wonder if the issue is related to the new Haswell processor and the CX600. In particular I'd read quite a bit about how Haswell chips use a lower power when idling and that "older" ATX PSUs do not necessarily deal with the less than 12V rails power very well.
Anyway, long story short, having rummaged through the motherboard manual I've disabled the CPU C states option in the BIOS, and having done so, so far, in the last 48 hours the computer hasn't restarted once.
I'm going to find some time at the weekend to run through and research the C states more thoroughly, determining which state is the more likely culprit, and whether I really need to worry any further but so far this has been the only change I've made that seems to be a fix.
I hope that helps you, or anyone out there having the same or similar problems, and I'd be interested to hear how you managed to resolve your problem regarding this too.