I didn´t install any windows update on the render machines. Pretty much a “vanilla” Win7 pro 64 on both
Workstations and Slaves. WinServer2003 on our server. IP4 only. Max 2011 Service pack 1. 1GB Network.
Fixed Ip Addresses. Router is DNS.
We actually ran into some problems with it but Not exactely your problem, maybe this helps anyway.. somehow 
Turn UAC off completely on Workstations as well as Render Slaves. (Looks like you´ve done that allready)
The machine you are saving your IRmaps to, is it a server OS ?
Otherwise it will just be able to handle 10 connections.
I remember that we sometimes had problems with really large IRMaps (not really win7 realated)
slowing the network extremly down. We just made a local folder
on each machine like c:\IRmap_temp\ copied the (in this case) precalculated IRMap
to the Slaves and set this path in the rendering dialogue.
Some clever workmate decided to register Vrayspawner as service,
resulting in an invisible Spawner after booting the machine.
People went to the machine started the Spawner as usual, so 2 instances were running.
In this case it will render without loading any maps (maybe same problem for IRMaps ?)
But maybe there is really something physical broken with your network.
Try changing network cables. Unplug everything not needed, change the ports on your router/hub
and try to narrow down the problem step by step.