I'd just return it and try another one. I'm one who's guilty of thinking I can fix everything. There comes a point when you put enough hours in to it to just accept the fact that it's probably a bad piece of hardware and not fixable.
I agree with BigFeet, I did ask you at the very beginning as to where you purchased the GPU, it was/is most likely a fake or a broken piece of hardware in the first place!.
Get yourself a different card as the HD640 is now legacy hardware anyway, I have a Radeon RX 570 which is very good and doesn't cost the Earth on eBay or Amazon!.
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