It may be something related to AppleGraphicsPowerManagement, try to add your card's ID under the model identifier you are using in Info.plist
[color=#FF0000]MacBookPro6,2[/color]
LogControl
0
Vendor10deDevice[color=#00BF00]0df0[/color]
or use a legacy kext.
This is the one I use
LegacyAGPM.kext.zip
You may want to test the original threshold values, I modified them to workaround a lag issue when my card switches from/to lowest state.
http://www.insanelymac.com/forum/index.php?showtopic=254053