gut feeling on reading the above which is a very nice rig is power , the nvidia stated limit for a founders edition is 650 watts whereas aftermarket cards can draw even more power with 3 x 8 pin connectors and your running a respected brands 750 watter but it feels a little low in juice for such a well specced machine , case fans as well can make an impact with their draw as your machine is keeping nice and cool so cooling is working well at those temps but that also enables a card like the 3090 to run higher clocks with its power/clock curves deciding how fast it can run at given temperatures which then leads to it drawing more power when it sees it is keeping cool so in theory it could demand a power increase near to max by virtue of the decent cooling
possibly try a 1000 watt psu , no bsod or error reports point as well towards instantaneous shutdowns which a leading cause is power supplies even corsairs , but also try swapping memory sockets around if your 100% certain on the power , other possibles … swap your usb connections around as this usb disconnect on amd systems may have a hand in your problem