Have a computer thats keeps shutting down

shutdown randomly. i suspect its the PSU, but i’ve heard it may be the memory? it will go fine for awhile and then bam …and after it shutdown ive tried to reboot and it shuts down even quickier..if i let it sit for awhile it will reboot alittle futher but will still shutdwon. all the temps are ok CPU+MEM don’t ever get above 40 C. the GPU runs bit warm but doesn’t seem worrysome. all help greatly appericated thanks. Thanks again, for all who replied!