You claim you bought one card from Gumtree? Can you remove the card that you originally had that you RMA'd and try the other card on it's own. See if that gives you the same issue.
That would be my first step in trouble shooting, try both cards on their own first. See do they both work ok. Then try them both in the other PCIe slot on your motherboard and see do they still work ok.
Then try them both together.
But, if I was you, after reading through some of your other problems, I would backup all my stuff, format my C: drive and do a completely clean windows install. And I also think I would run a memtest.
I would do this, back up your stuff, and do a format and a clean install of windows and keep every thing at stock and test one card at a time starting with the card you didn't RMA and take your time testing.