Lost Network Connectivity

Lost Network Connectivity

I am having an issue that I could not explain myself in the entire of my
career. The issue is when the entire computers just randomly and suddenly
lost connection to the server, running Windows Server 2008 Small Business
Server in VMware ESXi 5.0 U 1.
I originally think this is networking issue, where at the first place I
believe that my 3 Cisco Switches SRW2024 log the UPnP overflow. The UPnP
overflow can cause the connectivity dropped. When the entire computers
drop the network connectivity, I did try to reboot all Cisco SRW2024
switches. However, it did not fix the issue.
The weird thing about this, any computers that lost the network
connection, they still can ping and browse the Internet. They lost all
Network Drive and all computers freeze.
At the moment, when this problem happen, the only way to fix it is reboot
my SBS server. FYI, my SBS Event Viewer Logs did not tell me anything
about this issue. From the server side, it looks OK, has no problem and
working as normal.
This has happened in the last 3 days, and I have no clue what is happening.
What I have done to troubleshoot:
Use WireShark to monitor for what is happening when the computers drop the
connection. As best I could examine the result, and I could not find
anything wrong when computers are dropping the connections.
Some stage, the SBS server logs warn that my DHCP scope is 83% full. I
don't think this is the issue. To eliminate this is as the possible issue,
I turned off my Wireless Access Point to reduce my DHCP server distribute
IP Addresses to all mobile devices.
Check all HDD capacity in my SBS server and everything looks OK and plenty
of space, as well as my 4 Datastore in ESXi 5.0 U1.
Apart for those above, I could not find a definite answer of what is
happening. I need help on how to troubleshoot the issue.
Thank you in advance for clue and assistance about this.