Thanks for writing and for your question.
Unfortunately using these events in the context of a ScrollViewer based control is a tricky thing. As you may already know, with the introduction of Windows Phone 7.5 (Mango) Microsoft have optimized the scrolling experience by creating a new Manipulation thread where the input first goes for handling and is forwarded to you in given time intervals, not constantly.
This is explained here in a good manner:
So, you can either turn this entirely off and have the glitchy scrolling, or have the optimized scrolling and do not rely on the deterministic behavior of the manipulation events.
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