Martin,
I am experiencing the same problem with frame rate drops after 5~10 minutes. Here is my desktop setup.
OS:
Windows 7 Ultimate 64-bit (Clean Install)
Windows update to the latest 5/31/2010
Anti-Virus
Microsoft Windows Essential
Hardware with Driver
Black Magic Intensity (3.6.4)
Creative Sound Blaster X-Fi Titanium PCIe (2.17.0007)
DTS Connect Pack for Sound Blaster X-Fi Titanium series (1.03.08)
Dolby Digital Live Pack for Sound Blaster X-Fi Titanium series (3.00.06)
ATI Radeon 5870 PCIe (10.5)
Gigabyte X58-UD4P on board LAN card (Default Windows 7 Driver)
Hardware
Intel i7-720
Gigabyte X58-UD4P (Latest Intel RTS and INF updates)
12GB DDR3