Weird Automatic Network Disconnection

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.

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

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.

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