What are your system specs? Especially your power supply. Sounds like you overpowered it with those cards.
The psu is 480 W, recently baught, and the specs for the card 560 GTX say 400 W should suffice (on the box) (the older computer I also used to test the card has a 420 W PSU). The fact is that it was working for months, and it stopped working a few days ago. Could it be related to a PCI-e slot problem? I hope not!
Thanks for your answer.