Hard lock ups mostly while gaming (heat related?)

Hi everyone, I'm hoping someone might be able to give me some insight.

I built a system last September with the following specs:
CPU: Intel Pentium G3258 3.2GHz Dual-Core Processor

Motherboard: Gigabyte GA-Z97X-SLI ATX LGA1150 Motherboard

Memory: G.Skill Ares Series 8GB (2 x 4GB) DDR3-1866 Memory

Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive

Video Card: EVGA GeForce GTX 760 2GB Superclocked ACX Video Card

Case: Corsair Graphite Series 230T Black ATX Mid Tower Case

Power Supply: Thermaltake SMART 550W 80+ Bronze Certified ATX Power Supply

Optical Drive: Samsung SH-224DB/RSBS DVD/CD Writer

Operating System: Microsoft Windows 8.1 OEM (64-bit)

Around 4 months ago (February or March) I started having an issue with hard lock ups most often while playing World of Warcraft. But, it has also locked up on me a few times while just web browsing. The lock ups are almost impossible to predict (has happened during cutscenes, and while out simply traveling from point A to point B by myself. No particular content or spot in the game). They started happening once every couple of weeks, and have been seeming to happen more frequently now, about once per week. Windows Event Viewer shows nothing unusual immediately before the lock up. The computer just stops responding but image stays on monitor, no sound, fans and lights remain on. I have to hold down the power button to reboot. I do recall one time I lost the image, too, and monitor said No Signal. But that was only once. Most of the time, image stays frozen on the screen.

I have done the following to try to diagnose/remedy the problem (in this order):
-Rolled back video drivers to previous versions (have had several updates since problem first began, no change)
-Attempted to update motherboard drivers (their utility on the website failed to update it, and I couldn't figure out why)
-Thoroughly cleaned out dust, reseated GPU and RAM
-Reapplied thermal paste to CPU, stock CPU cooler
-Replaced PSU
-Ran memtest86 (4 passes showed 0 errors)

I am running HWMonitor while web browsing and while playing WoW. Everything looks to be within normal parameters (not sure about voltages, though. I don't know what to look for there), but one thing jumped out at me: the GPU max temp was about 81C while playing WoW. That seemed high to me, but a Google search showed that as long as it stays below 90C it should be fine. The GPU usage was 99%, and the GPU fan only ran at 73% max (there are two fans on the GPU, but only one value is reported?).

I have played around with my settings in WoW trying to reproduce the problem. Last night I had the Nvidia GeForce Experience program set the recommended settings for me, and it put mostly everything on ultra. I played it for about an hour without a problem, but could hear the fans working harder than my usual settings (which I have turned down to "good" for most things, except liquid detail and view distance at "ultra"). The max temp of the GPU was 81C. Then I turned everything down to "good" (which is the halfway mark on the slidebar) and played for a while again, and everything was fine, but the GPU temp still went up to 81C with the lowered settings (after resetting the values in HWMonitor).

Is there anything I am missing? Anything else I should try? I am at a loss, especially since I can't reproduce the problem consistently. It seems like it happens under heavy GPU load, but again, it has happened while simply browsing the web, also (not even playing videos).

Thanks in advance for any help you can give me.