Can you please post a dxdiag log?
It sounds like it could be a heat issue. You'll find some tools below, which can log your CPU and GPU temperature. Please log until you experience the slow down.
dxdiag
Windows XP: Go to your start menu and click on run. Type in "dxdiag" and hit enter
Windows Vista / 7: Press the windows key on your keyboard
or click on the start menu. Type in "dxdiag" and hit enter.
Now click on the button that says "save all information".
It will prompt you to save a file. Save it where you can find it.
Then
zip the file and attach it to a reply in this Thread.
Logging your CPU and GPU Temperatures
CPU: Download
Core Temp. Start it and activate logging in the menu under "Tools/Logging on".
The log file created is called similar to "CT-Log [TIME][DATE].csv".
GPU: Download
GPU-Z.
Start it, select the "sensors" tab, and activate "Log to File" at the bottom of the window.
Choose a location for the log file.
Zip the files and attach them to a reply in this Thread.