You'll also want to re-install the drivers, clean sweep, each time you swap the config around to be sure.
Check to make sure that you have enough power, these things are rated at ~250W each, when not overclocked, and theoretically can pull up to 375W from the supply.
If that single card is showing signs of issues, make sure it isn't overclocked or over temp when you run it. Use MSI Afterburner or Hardware Monitor to keep an eye on temps. If it was abused it might need the heatsink compound replaced, or just be damaged.