Hi Andy!
I've done some testing now with loadUI 1.0.x and I can confirm that your project behaves weird with very high loads. I also found that it seems to be working better when using the Fixed Rate Generator instead of the Fixed Load. Also, don't set Max Internal Threads to more than you need since loadUI won't necessarily reuse threads until it has to.
That being said... In your loadUI project, one TestCase was set to a fixed load of 5000 (that is 5000 simultaneous requests) for 11 minutes. This is an extremely high load (for example, Google handles ~34k searches/s. Each search takes about 0.15 seconds, giving 34k*0.15 = 5100 simultaneous requests -- and Google's got more than a million of servers).
The good news is that I also tried to run your project with 5000 simultaneous requests in loadUI 1.5 (internal build) local mode without any problems. Distributed to agents, I could multiply that number easily.
Regards!
/Henrik
eviware.com