If you swap cards and it works fine then it most likely the cards fault. Did you try to reinstall drivers? What was the 4870x2 temps? Its possible your psu is failing when 4870x2 is drawing power. Have any spare psu you can try out? You could just rma the card to the manufacture if your covered which i think you might be since the card isn't to old and most manufacture have 2-3 year warranty. If all else fails what is your budget for a new card? But try all the things i have stated i would hate to see a 4870x2 and your money go to waste.