Are you saying that it begins to do the 15 second time-out style shut down? If so, that's most always an indication that power was temporarily lost to the dock, or between the dock and the device.
Under Menu / Settings / System / Battery Settings / flip the toggle for "Sleep automatically when disconnected from power" to the left, shutting that feature off. If your device behaves normally after that, then you know it's an intermittent power issue. And don't forget to switch it back or use the power button to turn it off manually if you have it in that mode or the battery will eventually be drained.
Once having determined whether it's a power issue, we can work from there.
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