@AfroDite26 Ultra settings would definitely put more of a strain on the graphics card relative to the rest of the hardware, so it's reasonable to start with the card. It's not overheating or downclocking for no apparent reason, and it's not running at or close to full load most of the time either, but that's not definitive proof it's fine. I mentioned the PSU's 12 V rail: that's what supplies the GPU with its power, so if the voltage drops are in fact a problem, the graphics card would be the component to suffer for it.
If you have a different card that you could borrow, that would be the most straightforward test. If the game still crashes with another GPU, then the issue could be the power supply to the card, or the PCIe slot, or perhaps one of the connectors, but you'd have narrowed things down significantly.
Otherwise, you could get a partial answer by playing on the processor's integrated graphics chip. If the chip is currently disabled—you can look under Display adapters in the Device Manager, it would be listed as an Intel HD Graphics 4600—you'd need to either remove the dedicated card or change a setting in BIOS to enable the chip and disable the card. Then you could plug your monitor into the motherboard. You'd need to turn down the graphics as well, probably to low or low-medium. This doesn't eliminate as many possibilities, but it's a reasonable place to start.