Weird Automatic Network Disconnection

[sunday11a]

I am on a shared LAN and surf internet using it. Natting is used for internet access
Today i am seeing a weird thing happening
My box is win2k pro sp4.
After some time like 10 min or 20 min, the pings to the gateway stop.
They get restored after i restart the whole system or
If i change the IP address of my card.
As soon as i change the ip address, the replies are back. Then i again change it back to original and after some time problem is back
It happens even when my conenction is active, i was downloading a file and in between the connection broke.

This has happened sometime back also but not with such frequency. Like in a day once it happens, and that also not regularly.

Earlier i didnot have a firewall. Today i installed zonealarm security suite trial version, before this problem started occuring.
I have checked with HJT for any unknown app in startup but nothing unusual. I have also checked for running processes but nothing unusual.

What could be causing this.
The firewall or some other issue
Even while writing this the connection broke once.

[dinowuff]

Check your system logs, another pc may be loging on to the network with the same ip address

[sunday11a]

Checked the logs nothing suspicious
Also only pings to my gateway go down.
But pings to other systems remain up.
Could it be some problem with the gateway router.
I changed my local ip and checked for pings and after 15-20 min the pings are again down.

[dinowuff]

Did you mean to say that the problem started AFTER you installed zonealarm?