SMB fails on Windows Server 2003

Hi,

We have three IIS webservers. Each has two nics (Internet and LAN).
The servers are load balanced on their internet interface (port 80 only).

Server 2 has the website sources (classic ASP) on disk.
Server 1 en 3 get their sources from server 2 over an UNC path
(\\ip\share) over the LAN, set in IIS.
This is not ideal, we know. The sources will be replicated in the future.

Old situation
Server 1: Windows 2000 Advanced Server
Server 2: Windows Server 2003 SP1
Server 3: Windows Server 2003 R2 SP1

In the old situation everything was fine.

New situation
We re-installed server 1 with Windows Server 2003 R2 SP1.
Now each time after running a day or so, SMB traffic on server 1 only
over the LAN fails. This causes IIS to hang.
ICMP traffic is still possible though.

Tricks like disable/enable or Repair don't help. Only a server reset.

All servers have an Intel Pro/1000 MT Dual Port adapter.
Driver versions:
Server 1: 8.7.1.0 4/27/06
Server 2: 8.5.14 3/8/05
Server 3: 7.2.17 8/14/03

The website has up to 40,000 concurrent visitors.
Network load on the LAN is always very low at 0.3%
There are no errors in the event log.
We tried sniffing the LAN with MS Network monitor.
There was a frame which included this SMB error:
STATUS_MORE_PROCESSING_REQUIRED

which looks to me more like a result of the network failing.

We have tried to revert to driver 8.5.14 on server 1 but setup has
failed. This server is currently down.

Meanwhile server 3 has failed once with the same problem!
We are lost. :-(