Certain website stopped respond to some visitors

[monolith]

Hi, any help would be greatly appreciated

We have 2 web server running windows 2003 with nlb in unicast, with 5 websites running each under its own application pool and its own ip address, on occasion certain website stopped respond to certain visitor (some visitor are still be able to access the website), while other websites hosted in the same server continue to work.

I used network monitoring tool and found out that whenever the website stopped working, it stop sending and receiving ARP request, letting visitor continue to request without a respond.

We are using sonicwall, the two web server each have 2 nic cards, first one is for nlb and outside traffic connected to dmz zone, the other one is for internal traffic to file and db server.

Thanks