I've never had an unexpected reset on any of my windows phones (trophy / 8X). If you did a hard reset and it still happens then I'd suspect a fault on the circuit board somewhere. I'd also recommend the Lumia 928 if you're due for an upgrade
I have an HTC W8 phone (WP8X). There appears to be no rhyme or reason for the shutdowns. Sometimes, I'll just reach into my pocket and pull it out, press the power button - and realize it's OFF, when it had been ON when I deposited it earlier. Other times I'll be actively using it (typically Pandora) and it'll just power-down - I do see the "Goodbye" screen. It doesn't vibrate or make noise when this happens and it's sudden. I would say the phone shuts off about once daily, if not more frequently than that. I refuse to use the phone as an alarm clock because of this and it has resulted in a multitude of voice mail, which I'm not pleased with as the phone is my only line.
Do these symptoms sound similar to your power-down issues?