I've to correct my last posting. The solution was not really stable.
But now I found the reason: The BIOS setting for CPU C State Capability [Cn]
With C3 it works fine but with C4 it worked weak as described.
My hardware: ASUS P5N-E SLI with newest BIOS, Intel Core2Duo E8400, 4 GB RAM, ASUS GeForce 8600 GT and enough HDD space connected via SATA
My software on HOST: Windows 7 64bit, Kaspersky IS 2011, VirtualBox 2.1.6, and a lot more