System Freezes While Under Load

(This isn't a BSOD situation, but I can't find a forum section for system freezes)

I have an i7-5820K overclocked to 4.2GHz as well as two GTX 980s in SLI. Occasionally while gaming, the system will lock up and will not respond until I shut down the computer either by turning off the power supply or holding the power button. I asked around about this issue, and some suggested that I turn up the CPU voltage. My overclock was stable throughout an IntelBurnTest session, but I decided I'd give it a try anyway. I started at 1.15v and every time the computer froze I restarted it and turned the voltage up a little. I'm now at 1.19v.

Is this a CPU voltage issue? I know there is not a lot of information to work with, but I'm not sure what other information to give. No components get too hot (CPU doesn't really go over 75°C when under extreme load and GPUs top out at the hard-coded max safe temp of 80°C), so I wouldn't think overheating is an issue. This issue also occurred when I only had one 980.

If the system locks up again, I'll turn off the overclock and see what happens. If it still locks up, I'll move some GPUs around.