No network connection without apparent reason

Dear All,

I have an iMac OSX 10.8 connected to my router using an ethernet lan cable.
Everything is fine but sometimes there is no connectivity, the connection seems to drop.

I have tried the following:
DHCP, DHCP with manual address, manual address. Nothing changes, when there is no connectivity nothing is working. The DHCP does not renew, and placing a manual address it says there is a conflict on the network and that there is a device with the same ip (not true), and this happens with any _free_ ip in my network. In my network there is only one DHCP server working on a fixed range of ip to be released.
I changed also the ethernet cable with a new one. Nothing changes.
Nothing changes also rebooting the router.
In the past just rebooting the iMac or activating the wi-fi would solve temporary the problem. Tonight also wi-fi did not work.
In my network I have many devices connected both with manual addressing or working with DHCP and no issue with them while the iMac it is not working.

Than doing nothing after some minute or some hours it work again.

Does anyone has some ideas, advice, test, or experienced something similar too?

Thank you for your kind answer,
dk