I see what the problem is - your PC is overheating - you need to add some more air holes to it. As a guide, ideally the metal between the holes on the sides, front and back should be no more than 2mm wide, although if you use the top for a shelf then you might be OK with going up to 5mm wide.
Don't forget you need good airflow through the case, so make sure any large blocks on the inside have adequate vents in them, too.
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