Hm. Well I've got a similar proglem, but that's with a W850, which is a slide phone.
I got a tip that it could be the upper keypad on the phone, or the flex cable that runs between the two parts.

So maybe it could be one of the components on the phone that has gotten a short circuit. Perhapse you could try detaching one component at a time like the keyboard, and see if the phone still resets itself.