I had that exact same problem.
I took my iPhone to a genius yesterday. Boy was that a waste of time!
I explained it to him what happens to my phone so he ran the diagnostics on my phone and it showed up as 0 times my phone has restarted. So I told him well then it's just shutting off I guess or it's just totally unresponsive. He looks at me and tells me "oh, well that's a whole different issue." He told me it can become unresponsive because of some applications blah blah... and that I can hold the home button to quit them. I told him that's not it! He then unplugged my iPhone and handed it to me. I was surprised! I told him "That's it?" He looks at me and tells me, Yupp. I told him it is really annoying having your phone lock up/ shut down 2-3 times a day. He said he understood. Then I told him it sometimes happens after I text message people and it can be random and it happened once after I got off the phone with someone. He then tells me "I'm seeing a pattern between phone calls and text messages, It could be that some of your contacts data is corrupt and you will have to manually re-enter them into your phone." He also told me to restore my phone and set it up as a new phone. I told him that i've done that before and the problem still happened after that.
About now I was getting a little angry... lol
So I told him that i'm just going to go home and restore my phone again, set it up as a new phone and that if my phone continues to have this problem I am going to bring it in again. He tells me Definitely, we want to know what's going on with your phone.
If I didn't tell him "That's It?" He would have let me leave the Apple store with no answers at all!
I restored my phone, again, like he said and I didn't waste my time redoing my contacts. So far, so good. I just hope i'm done with that problem!!!
