I was not able to reproduce this issue.
Would it be possible for you to create a test project (TestList included) that runs against a publicly accessible Silverlight app? If you provide such a test project we can reproduce the problem locally and analyze the issue. I would suggest Telerik's Silverlight demo page for this:
Alternatively you can send us your actual test project but that would be less useful since we won't be able to actually run it and see the issue first hand.
If you consider any of the data I'm requesting to be too sensitive for a public forum I can provide an e-mail account.
One last thing to check is the setting for the TestList (screenshot 1). Check for any unusually long timeouts.
the Telerik team