When I try to connect my Dell Optiplex 755 machine to some 100 Mbps switches/routers I very poor network performance (about the speed of old analog modems). It is impossible to copy larger files over the network because it would take too much time and/or because the network connectivity drops quite often.
The only way to fix this issue I found is to limit the connection speed to 10 Mbps.
As far as I know this is a common problem not limited to Dell machines. The same issue was observed with HP machines in combination with Cisco 100Mbps routers (found on Google). Some people were able to fix the issue after severall mainboard exchanges.
I tried several driver versions and of course I have also installed the latest firmware updates available from Dell.
Does anyone know what causes this issue? Is there any way to fix it?