Nvidia Sentinel has detected that your video card is not getting enough power, and has lowered the settings automatically

Oh well (: Back to the drawing board.
Thanks again everyone.

This may sound a bit stupid/unclear, but does your motherboard have more than one power socket? I had a similar problem ages ago, the main socket was 4 pins short, but there was a seperate 4 pin connector to plug in another socket on the motherboard. Boy did I feel stupid.

Aye, used that one already. It is 6 pins, and has got a plastic blocker in it so it can work as 4 pins. The warnings are far less common with that power connector plugged in, but they are still there.