I would say try reloading your OS from scratch and see if that works. But thats a lot of work, best bet is to try the card in another computer. Is that possible? Very well could be drivers still left on your system corrupting your driver install. Did you try driver sweeper? I've had bad driver installs before that couldn't be fixed (by me anyways), and have had to wipe the system and start fresh which fixed the problem. Not saying thats it, but the problem with troubleshooting is you have a lot to rule out, and a fresh install is the best way to rule out software issues, leaving you to deal with troubleshooting the hardware.
If it's not the software, and it's not the card, it could be bad pcie slot, but if your old card is working then thats probably not it. Might not be getting enough power. Might need to upgrade your motherboard firmware, maybe it's reacting funny to the PCIE 2.1, which is supposed to be backwards compatible, but you never know. The only problem with that though, is if you mess up you could render your M.B. dead. The odds of getting 2 bad cards is low, but it's possible.