Modem or Motherboard Problem ?

Hi Experts, I have no idea if this is a modem problem or my motherboard ethernet controller problem =(

Problem goes like this:

1) I switch on my modem

2) Then I switch on my PC

3) For the first 5-10 minutes, I am able to access the modem gateway 165.192.254.254
I can connect to the internet as per normal.

4) 10 minutes later, I am unable to do so anymore. I have to configure IE settings to connect to the internet through my ISP Proxy Server.


I checked ipconfig under cmd, and the gateway is stated as correct.
I PING my ISP website and I get no data loss, 4 packets of data.
I PING my gateway and I also get no data loss.

I'm using MSI K9N6PGM2-V Motherboard and Siemens Speedstream 4100 modem.

Anybody knows what may be the problem? Thanks!