You must test each component separately. Try borrowing a power supply and see if it works, if it doesnt crash then change your psu. If the problem persists , try a different gpu.
Just because your computer boots into windows it doesnt mean the psu is ok.
We had this pc at the shop that it would boot fine(most of the time), but under any kind of load it would flick off. No warning, no crash. Fans keep spinning but no video. Turns out PSU was dying, I did the process i described above and it booted fine with a different psu.
PS.
what are your voltages under load?