Does/did any of you overclock your phone?

I am almost sure that the problem is your battery which has become defective from too much heat/too fast power drain. Overclocking is one of the sources to make the battery defective. The defect then shows (black screen, freeze, restart) each time the phone is reaching a certain temperature - either through usage of the phone (heavy cpu use makes the phone hotter) or by placing it in a warm place like a pocket close to your body.

Try buying a new battery (or lend one from a friend/collegue) and see if your phone works again.

It would be nice to hear your feedback...