PC shuts off when GPU power draw goes above 300w, why?

My cpu is pulling 80w. I'm trying to get my vega 56 to be able to run a game at 4k 144 fps, but to do it it needs to pull 350w. However, when it's running above 300w, my pc can randomly shut off. The higher the power usage the quicker the shutdown happens. It isn't immediate, it can take up to five minutes to happen. The gpu isn't overheating, reporting 75c temp. I have to hard reset it to get it to turn back on again. My PSU is a 650w seasonic prime titanium, which should be more than enough for this... why is this happening?