Update your Motherboard BIO's, if that doesn't help do the following
Try playing with 1 card and see if it works fine, then the other. if both work fine its not the individual cards. then try different positions on the pci-e 16x slots. if the same problem occurs(assuming you haven't found the problem already) try different SLi bridges.
If none of that works it could be OS/Driver issues, maybe clear out your drivers and do a re-install.