SLI setups are prone to issues with Microstuttering and other odd problems. Especially on games that don't perform optimally in multi-GPU configurations.
Make sure your PSU is capable of handling both cards. If the PSU doesn't supply more Watts than the computer needs, then you need to get a bigger PSU.
Also make sure that you did a clean install of the correct and more recent drivers. Check to see if the older drivers were cleanly deleted.
Do the cards have decent cooling? Check your GPU temperatures. If they are high (depends on who you ask, but I try for less than 75 degrees Fahrenheit), make sure the cards and the rest of the computer is dust free. Use compressed-air and clean it all off if you have to. Then, check to make sure...