Weird internet dropouts problem - CentOS 6.1

Hi, i am new to CentOS6.1 and also to linux world. I learned basics of linux few days ago on my windows machine with virtualized CentOS6.1 Desktop installation. Everything was fine, internet working - no problems. Then i changed harddisk in my pc and decided to install directly on it CentOS Minimal. I have configured NIC during the installation of CentOS to use dhcp of my router. Internet works, i have succesfully updated system using "yum update"(update was very slow, few kilobytes per second despite i am connected to 20Mbit fibre internet). Then i tried installing CS1.6 server. It started downloading server files from steam server but few seconds after that it stops and nothing happens, just like internet connection dropout, but there were no error messages and no return to shell. After half an hour of waiting nothing changed so i returned to shell using CTRL+C. I thought it was problems with steam servers but when i tried to ping another outside servers it happens a! gain. Ping start pinging, i've got maybe six answers but then it stops, no errror mesage, no return to shell only CTRL+C. Please where can be the problem? Thank you very much.

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