Incomprehensible system brakes?

Question

Iron

The situation is as follows, in the spring of this year, after launching Battlefield 1, my PC turned off. Nothing serious happened, I thought and tried to start it, to which the iron machine showed no sign of life, turned off the toggle switch on the power supply, waited and tried to start it again. He started, the motherboard squeaked about problems with the video card. Subsequent launches occurred every other time, sometimes only the 2nd monitor worked, sometimes the first one had red ripples. After the first time, the system began to slow down heart-rendingly, when talking in a conditional Discord, it was worth moving the cursor and the sound plugs immediately began (and begin), brakes when opening programs, etc. VLC player crashes when playing video, but not always. The old card still died, and I went to buy a new one, hoping that all of the above will disappear, but no. After installing a new video card, The situation has not changed. Games work only in dx12, at 11m the load on the GPU jumps, FPS, respectively, too. It’s better not to talk about frametime at all, it goes away in a second. Well, all the other joys of the brakes. As an experiment, I put 10k and 7k on other hard ones, everything is absolutely the same there. I drove the system with aida, furmark, there are no problems. In stress tests, everything works like a Swiss watch. memtest also passed successfully. There are no food cuts. Bios also updated, PCI-E tried to put in different modes, zero reaction. In stress tests, everything works like a Swiss watch. memtest also passed successfully. There are no food cuts. Bios also updated, PCI-E tried to put in different modes, zero reaction. In stress tests, everything works like a Swiss watch. memtest also passed successfully. There are no food cuts. Bios also updated, PCI-E tried to put in different modes, zero reaction.
System:

• Processor Xeon X5660
  
• M/B Supermicro X8DTU
  
• Memory 12 GB 1366 MHz (4+4+4) in 3-channel mode
  
• PSU evga 750bq
  
• Video Sapphire RX 570#4g connected via pci-e x16 riser.
  

The processor did not overheat, however, like other hardware, but there is no talk of overclocking at all, the server board.
Problems appeared after the first such freeze, before that everything was perfect. Could there be a problem in the motherboard, or some Chinese riser, or is it easier to throw out the trash and buy a new one? But then how to explain the failure of the old card ...
ps On the second hdd there is a poppy, and oddly enough, it works fine, that until the death of the old card, that after.