Hi Dave.

This sounds a lot like the issue discussed on page 14 of this "How to write a service broker" tech arcticle (http://www.k2underground.com/files/folders/technical_product_documents/entry21898.aspx):

"Important update August 14th, 2008: The previous version of this article included code that would not function correctly under load. In these circumstances, because the original version of this service broker example used static classes, service instances would share configuration data. If you based your Service Object on this prior example, please go back and use the approach taken here with a new property on the DataAccess class called "Binder" which enables the connector information to be bound to the service broker information. Other classes in this solution then use that binder property to retrieve connector information on a per instance basis that will perform as expected under load. "

You might try checking the source code for this service broker to see if this applies.