After some problems with the NIC driver, my Windows Server 2008 is running fine.
With all the BSOD’s i had in the beginning, the index was below 5.00 at some point. But since i replaced the NIC driver, the Sysytem Stability Chart is rising steadily.
No problems whatsoever, everything is working very stable. The only disadvantage of working with 5 virtual machines on Hyper-V is that when you have to install updates on the host that require a reboot, all VMs have to be restarted. Besides that, i can only see benefits of working this way.