Wireless OK, but wired is not!

I got this setup:cable modem, router, one PC next to the router wired and another PC and a laptop upstairs wireless. While both wireless work fine, my wired PC after 5-10 mins of handling internet traffic starts losing LAN connection every 10-15 secs and then reconnecting back. This effectively kills this PC's performance and also is very annoying. I have tried the following: replaced cable modem, got a different router, changed Ethernet cable - all with the same results. I have also played with both the router and PC settings, but to no avail either. I searched the Web for similar experience, but found none. Please, help me as I have run out of ideas. Thanks.

mabkin