Sporadic reboots/lockups while gaming

[Gyrael]

A while back, I made this thread. As you can see, I thought I fixed the problem by uninstalling EVGA's Precision tool. However, I still have some problems.

 

Sporadically, my PC will reboot or lock up (a small sound chunk of what was playing will repeat, no inputs or power button work, only reset button) while playing a demanding game. It's only happened a few times in the span of like 4 months, but yesterday I got a reboot and today I got a lockup, both times while playing Dota 2, so I'm worried again. It's also happened in Far Cry 3. However, I've also played other games like Dark Souls II and XCOM: Enemy Uknown for long periods of time without it happening.

 

I've also ran Prime95 and Furmark simultaneously for long periods of time (one time for over an hour, right now I've had it running for 45 minutes, other times for between 15-30 minutes) to see if it was a PSU issue once again, but it has never resulted in such behavior. I've also ran memtest86 for an entire night with seemingly no errors (although to be completely honest I don't really know what's going on on that screen).

 

My specs are the same as in that previous thread (running on the EVGA PSU now), but here they are:

 

Asrock Z77 Extreme4 (BIOS version 2.70)
intel i5-3570K @4.4GHz

2xEVGA 660 Ti

Corsair Vengeance 2x4GB 2133MHz underclocked to 1866MHz (it would corrupt my OS on stock speeds...)

Creative SoundBlaster X-Fi XtremeMusic

Seagate 7200RPM 250GB HDD

Seagate 7200RPM 500GB HDD

Kingston HyperX 3K 120GB SSD (OS)

EVGA SuperNova 750G 750W

Windows 8.1 64-bit

 

So basically I have no idea what's wrong, and I'm looking for any advice you all might have.

 

Edit: I said this in the old thread, but I'll reiterate that it's not temperatures as I've already checked those while stress testing and in game.

[Fulgrim]

Try running with just one of your graphics cards installed OP. If it persists, swap to the other and try it.

[Gyrael]

I would but since this is so unpredictable I have no real way to make sure it's working in any reasonable period of time.