This is definately an SLI issue, it can be a pain the arse to setup but its great when it works well. Here are a couple of things to try.
1:- Swap the cards around card 1 > slot 1, card 2 > slot 2, card 1 > slot 2, card 2 > slot 1
2:- Try a different SLI bridge
3:- remove all Nvidia drivers and reinstall them with only 1 card installed.
4:- In NVP, set single display performance mode, maximum performance power option. (EDIT - Try this one first)