Happened to me a couple of days ago, 100% stock everything, haven't even HSPL'd it yet.

I only installed google maps, and a couple of other programs. It reset itself when using the WM messaging app to send a text, as soon as I hit send, it restarted. The text still went through, though.

Only happened once so far. I -did- re-enable QuickGPS, which was completely disabled by default on the stock Sprint ROM, but why the restart while texting with nothing else open?