This is usually caused by the cards downclocking.  This can be monitored through EVGA Precision (www.evga.com/precision) you will see, when using the On Screen Display, that the poor FPS coincides with the lower clocks form the card going into power saver mode.  In most cases this can be resolved by going into the games option, the into display/video setup area and turning up multisampling.  The GPU detects a larger load and comes out of power save mode and the FPS will, in most cases, shoot up dramatically.