try to run the machine without the graphics card if you havent already
you could just be missing drivers, or something else
also, check if your cpu has any gunk under it since you de lidded it. If you haven't placed any thermal compound, or there is excess that hits the other parts of your processor, try as much to remove/add thermal goop.
also, you can check the total wattage of your machine, and see if your power supply is too weak.