Why is my PPC resetting?

[Guest terrypin]

I have an HTC Touch Pro 2 running WM 6.1.

Typically I leave it re-charging from my desktop PC via its USB sync cable, with the display off. IOW, in what I believe is called 'hibernation' mode. Normally when I press the power button it then starts up from wherever I left off (usually the HTC Home screen). But recently it's started re-booting instead, as if I've done a soft reset. I haven't narrowed it down to a precise time period, but this probably happens if I leave it for an hour or three (and always happens after overnight charging).

Presumably some software is responsible. How can I most easily discover what please?

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Terry, East Grinstead, UK