RPC on Windows 2003

Hi,
our group develop a client server application that uses RPC as the
communication mechanism. Due to the nature of the application we can
have RPC calls blocked at the server for a considerable time and there
may be may be several thousand RPC calls outstanding to the server at
any one time. This application has been running on Windows NT, Windows
2000 and Windows XP for years and we are easily able to scale to 5000+
concurrent RPC calls on these platforms.
Mow we are starting to have deployments on Windows 2003 and when
scaling these we have problems when we have approx 1700 RPC calls
outstanding. Any attempt to create additional calls results in Windows
1130 error. "Not enough server memory". We have tried the IRPStackSize
change seen in many postings on the net, but no joy.

Can anyone shed light on any parameters that can be set to make Windows
2003 RPC subsystem scale to at least the same levels as Windows 2000?

Cheers Simon...