WF4: Workflow service stops accepting new requests

Question

I am hosting a small workflow via the WorkflowServiceHost class.

using (WorkflowServiceHost wfHost = new WorkflowServiceHost(new QueueWorkflow(), new Uri("net.pipe://localhost/workflow")))
{
    wfHost.Open();
    Console.ReadLine();
}

This workflow starts via an incoming WCF call (currently NetPipe binding) using a Receive activity, proceeds into a custom activity does some work and then creates a bookmark and goes idle (does not unload).

All this worked well at the beginning, however when I have my load tester set up to start 1000 workflows, 928 workflow instances start and proceed up to creating their bookmark, the remaining 72 block either in WCF or in the Receive activity.

• The test always stops at 928 workflow instances. (A workflow memory restriction maybe?)
• Resuming one of the workflows and allowing it to run to completion allows the next workflow in the list of blocked starting requests to begin.

Any ideas greatly appreciated.