Network connection from server to firewall stops every few minutes

Hi! I have a strange problem happening on a server:

The server looses its connection to the firewall (And the Internet), and the only way to fix it is to disable and enable the network card. It happens with varying frequency. Now every 1-5 minutes, earlier today it was every hour or so.

When the problem occurs, all other servers and computers can reach the server. They can also reach the firewall and the Internet, so it seems the only thing that happens is that the specific server and firewall no longer talks to each other. There are no error messages in the logs that I can relate to this.

Continuos PING reports normally 1ms respons from the server to the firewall, but sometimes it goes up to 10-50 for a little while. In the mean time, another server has steady ping-response of 1ms to the firewall.

I have updated the drivers.
I have updated HP Network Configuration Utility.
I have changed the network cable.
Everything has been restarted.

The server is a 2 year old HP Proliant ML350 G5. Network card is HP NC373i (Broadcom 1GB), and the firewall is a Juniper SSG5 (Netscreen).