Hi, and thanks for your reply.
VMTools are up to date, ESXi Version 5.1
I could just locate the cause of the issue. The energy-options of the Win7-VM were not on "Always on". First I tried to switch the PCI-Express Option in the energy options to always on, to see if that causes the issue, but it hasn´t done that.
So I diabled everything of that stuff and now it seems to work.