|Something else to try...|
This is a desktop/tower system, so
Power down and remove the mains cable - so no power is connected in any way.
Remove the graphics card and clean the edge contacts - using a soft pencil eraser. Ensure no bits stuck to the card edge connectors, and re-install the card. Then remove and re-install again.
While you're at it, remeve each RAM stick and clean edge connectors as above. Re-install each RAM stick at least twice (as above with the graphics card).
Power up again and if fault persists?