A few more information:
PC 1 (Desktop):
Windows 7 64 bit Home Premium, atm I'm using the 12.4.39 driver.
(DHCP, tried also static IPs)
PC 2 (Homeserver)
ESXi 5.1, standard driver, several VMs (Linux and Solaris)
ESXi host has static 172.27.17.5/255.255.255.0 the VMs DHCP or static
I'm sure, that the ESXi has no firewall that block pings, the VM I used for the tests also not.
Gateway/Router/Switch
TP Linkk TL-WR1043ND (with DD-WRT)
DD-WRT has never made any problems for me.0
I tried deleting all other Networks but that didn't solve the issue.
The difference is also, that I can connect to LAN, except the one that is also using the Gigabit CT.
I have the issues on Linux and Windows too, so I don't think it's an driver issue.
When I have a little bit more time I will look into wireshark and try a direct connection without Router/Switch.