3 year old PSU cause of my reboots?

[FlyingPig]

Got myself a new graphics card (GTX 970) and I'm finally doing some proper gaming. I've been getting random reboots after gaming for a few hours. Could it be my CPU overheating? Or a dodgy PSU?

My specs:

Motherboard P67A-GD53 (MS-7681)
i5 2500K CPU (overclocked to 4.2GHz)
OCZ PSU 700W Stealth Xtreme
8GB RAM

I'm fairly new to monitoring while gaming but it seems the CPU temps stay around 70, though I guess it could go higher and then the computer reboots to protect itself?

 

[FlyingPig]

Thanks. I did get BSOD when trying to reapply the original 4.5Ghz overclock but in the end settled for the easy way out using the MSI Overclock Genie tool, which sets an automatic, moderate OC. Haven't had any BSOD since so I suspect it's not that.
I'll download these temp monitoring programmes and see what I can find out. It's only happened a few times but obviously need to keep an eye on it. My CPU is definitely the weak link in my system now, just hoping I can get away with it for another few years.

 

[FlyingPig]

Just happened again after playing a game for about an hour. CPU temperatures got into the high 70's towards the end, but from a bit of googling that doesn't seem too problematic.
What I've done now is reset my CMOS battery (change pins and taken out battery), something I hadn't bothered doing after flashing my new bios (which I had to do to make the GTX970 work on my 3 year old motherboard) and which I was warned about could give me issues if I didn't reset.
I was thinking: could it be the power supply to my graphics card? Before, on my GTX 570, I had two 3-pin connectors, but it seems a bit different on the 970. I used the same cables but maybe I should have used the provided 4-pin?
See the picture of how it is connected now (the separate smaller one of 2 pins is not connected at the moment, should probably do that?).

 

[FlyingPig]

Oops, silly me. It was a bit awkward to fit but I managed in the end. Fingers crossed this or the CMOS was the issue.
We live and learn

(manual is quite vague by the way and mentions 2x6pin connectors but shows the 8pin)