I have Optimus on my machine as well and it can be a real pain in the butt. Under the nVidia Control Panel, go to Manage 3D Settings, and then under the Global Settings tabs. Is the GPU set as the Intel, nVidia, or Auto-select? There should be a way to globally force-enable the laptop to use the dedicated GPU all the time (let's be honest, no one's crazy enough to run a dedicated GPU on battery power) by setting it to the "High-performance nVidia processor". While the temperatures may get a little warmer, this way you don't have to worry about the Intel IGP kicking in. Also, try the most recent 310.70 drivers.
Custom Desktop -
CPU: Intel Core i5 3570 3.4 GHz to 3.8 GHz (Turbo Boost)
GPU: MSI nVidia GTX 660 2 GB GDDR5
RAM: 8192MB DDR3 SDRAM PC2 10600 1333MHz
OS: Windows 7 Professional 64-bit
HDD: 1 TB Western Digital 7200RPM HDD
CPU: Intel Core i5 3570 3.4 GHz to 3.8 GHz (Turbo Boost)
GPU: MSI nVidia GTX 660 2 GB GDDR5
RAM: 8192MB DDR3 SDRAM PC2 10600 1333MHz
OS: Windows 7 Professional 64-bit
HDD: 1 TB Western Digital 7200RPM HDD