It's been ages. And I should probably have done this before but... anyway.
Turns out it was the Motherboard. I got a replacement and everything is working fine and dandy. Thanks guys!
Yeah, so my CPU is fluctuating anywhere between approx. 1500 MHz and 4300+ MHz... along with CPU Fan speeds fluctuating between approx 881 RPM and 675000 RPM.
Ahh, okay. Considering I can't run it anywhere near as hard as I should be able to just to check is making it very hard. I'll try uninstalling my GPU driver and running intensive stuff.
Just downloaded the previous version and tried it and it seemed to be even worse... might just be coincidence, but it didn't even let the option screen fully go away once I selected a test to run.
Okay, so I'm running Memtest86 with a different USB stick. This time I chose the non-UEFI option and it's completed a full pass with no errors, and it's not resetting on Parallel Mode like it was... So... PSU?