that did not work what did was using the network assistant to to diagnostic the wired network connection it found nothing wrong with it but fixed both wired and wifi, checking the wifi connection did nothing by the way.
Same problem for me (Late 2009 27?nches iMac), Network is OK for 1 min or 20 after reboot and go down. With Safe mode StartUp, it can last 2 hours but everything is slow.