A few weeks ago I made a new build MSI Z87-G45 Gaming motherboard with an i5-4670K, a Cooler Master 500W PSU and only one Patriot Signature 8GB DDR3 RAM stick.
Last month everything has been working, kind of. Sometimes I can play games that max out the CPU (I have no GPU installed at the moment) and everything works just fine, but other times I can open two tabs in Firefox and everything will simply crash, BSoD and reboot. It will stay up for 5 days nonstop if I'm lucky, and sometimes it will reboot 10 minutes after loading the OS (both Windows 7 and Gentoo).
The most surprising(?) thing I've seen so far is when it randomly freezes for about 3 seconds, the screen goes black and starts flashing bright colors at a rather high pace (maybe 10 flashes per second).
Memtest86+ passes with no errors (~8 hours) and the small/in-place FFT tests of Prime95 run for at least 24 hours with no errors, but the Blend test fails within ~5 minutes if I set the memory variable to 7000MB. I tried putting the stick in another slot, and now it fails within ~6 hours, which is an improvement but still. The BIOS has been updated to the latest version as well, as it came with an old version (v1.1 -> v1.4).
I have absolutely no idea what's going on. This stick seems to have rather high latency (CAS 11, not sure about the other timings), but swapping slots made things slightly better. I also have no way of testing other RAM sticks because this is the only DDR3 stick I have. Everything is running at factory settings, the heatsink (CM Hyper 212+) keeps my temps below 60C at full load and everything is brand new, no open boxes or refurbished parts.
tl;dr PC rebooting/crashing randomly. Temps are fine, no overclocking, memtest passes, Prime95 fails on Blend but passes the other tests.