So last night I went to bed and left my PC going so it could download overnight. This morning I turned it on to find it in the Windows 7 partition. This means at some point in the night it's actually restarted, since it boots into Windows 7 not 10 by default.
So I restarted (by the reset button IIRC if that makes any difference) and it wouldn't go past the bios. Once it hits bios it restarts and continues to do this unless I enter bios or turn it off.
So far I've removed the cmos battery and moved the jumpers, tried each stick of ram on their own, disconnected the reset button (you never know..) and removed the GPU. The only difference I've seen so far is when I removed the GPU, using the onboard graphics the signal is there but after bios it's just a blank screen. I have no idea what it's doing then.
Specs: Intel i5 Haswell, GTX970, 8GB DDR3 ram, Superflower Golden Green HX 450W PSU (how the bloody hell does that power everything?!)