network drops after a while of using

my network acts fine when I start the computer but after about 30 Mins the speed goes down gradually and drops the connection finally
even local file sharing respond slowly
it's showing fine in right lower corner next to the clock
troubleshooting shows "DNS not responding"
using custom DNS server didn't fix it
pinging the gateway loses 25% of packets
pinging the localhost is fine
format didn't fix the problem

now running: custom built PC - Win 7 ultimate x64 bit
Ethernet: -built in- atheros AR8151
No wireless adapters

any ideas ?

1. Is this a wired or wireless connection issue?

- Wired

2. Who is your Internet Service Provider (ISP)?

- SA provider: STC (saudi telecom company)

3. What type of Broadband connection are you using?

- DSL

4. What is the exact Make and Model of your Modem, Router or Modem/Router Combo

- AFAQ SHAMEL - 007. (HG532b)

5. What is the Name of the Anti-Virus, Security or Firewall Software

- Avast internet security purchased from www.Avast.com


*note: the problem appear just for one computer
and the internet is working fine for other devices

thanks for everyone who replied

I've tried changing cable and port in the router but didn't work for me

about the wireless as I've already mentioned that I don't have one

I borrowed ethernet card from my friend and it's working fine so LAN port problem proved


Mr. 2xg thanks for responding
my broadband is not heavily utilized as my speed test in all devices is between (450 - 390 KB) for downloading and (24 - 19 KB) for uploading

and the latest driver is already installed

no lose of packets in safe mode

scanned results are negative (no infections found)
bought new Ethernet fixed my problem