I use MSI Afterburner, tracks FPS, power, fan speed, usage, temps, and core speeds.
I dont currently have BF3 so I cant check. My best guess is the card wont get enough power, so it then lowers its clock speeds to draw less.
Or a thermal barrier somewhere.
Im going to try swtiching it out for a 750W, and im getting a Xigmatek Gaia tomorrow which im going to use to OC to (hopefully) 4.5Ghz. See the cooling in my case (haf 912+) is incredible, i have 5 fans, one of which is a 250mm on top, two 120mm in front, 1 120mm on the side, and 1 140mm exaust. So i was able to oc it too 4.1Ghz with stock fan and it still sits at 51-52C in bf3, so with a cooler im sure i can get it awfully high.