Imac looses Ethernet Connection

Question

The Problem:
We have now 3 imacs with ML 10.7/8 who started to get Networkproblems since 2 weeks.
When you start the imac, you could work for 5-10 Minutes, and then you have no connection to the network.
No error Messages appear. You must reboot the Computer and then you could work for 5 Minutes.
The only thing i saw at the console is that the interface following messages:
kernel: en0: promiscous mode enable failed.
and then same message with promiscous mode enabled
These Macs worked since 1 Year ´without Problems.
We have same imacs with ML 7/8 on the same switch in the same vlan without any probs.
The Environment:
The macs are connected via Kat 5/6 cable to a switched Network with vlans.
What we tested:
We changed, the cable, the port, the switch, put the computers to another location.
We changed from DHCP to Static IP.
No Result!!
What we noticed:
if you start the imac in save mode, you could work, but this is no solution because paralles did not work.
Put the mac in another vlan it works. But in the origin vlan there are no errors or problems, other macs with the same configuration works without any probs.
Any Solution ?

Answer 1

First update OS X to 10.8.4, the restart the modem and router. If the router has een used for a few years then check to see if it has the most up-to-date firmware.