I have the exact same problem, after a couple of minutes of playing, i get a huge slow down and CPU usage jumps to 100%, for many games like SSBM, SSBB , but i guess it is a problem in the new builds
Windows Vista Home Premium 32 | Intel Intel Core i7 720QM 1.6-2.8GHz | 8.00 GB of RAM | ATI Mobility Radeon HD5870| CrossFireX1TB Raid0 7.200rpm HDD | Bluetooth 370 | Intel WiFi 6300 | Windows 7 Home Premium