RPC Unavailable using HTTP post request

[Grant Smith]

I have a .NET web srvc which internally uses DCOM to
connect a remote activex exe.

This works fine from clients provided connctions are made
via .NET web svc classes, clients connecting using HTTP
POST calls and browsers on a certain clients site receive
RPC Server Unavailable error creating the DCOM object
remotely as soon as more than one concurrent request is
made - my test harness which spins up 100s of threads
NEVER receieves this error but its using .NET classes to
instantiate and send requests to the service.

This is not an issue internally - only at one clients
site?

Any ideas or config settings that may need to be altered?