I've been tearing my hair out with this problem for the last couple of weeks, it is so random, I could be sending a text, reading a book, or just speaking to someone and the handset just switches off by itself and reboots. The problem is over the last weekend it has started doing it over and over again for about an hour without me even touching it, I just sit there and watch it doing it! I have tried everything, taking the battery and sim card out and leaving it for an hour. Trouble is it is so random and works fine for a couple of hours or so and then wham it does it again. I am heartily sick of this phone; I went back to T-Mobile yet again! and they gave me a replacement battery to see if that made a difference, in other words they haven't got a clue either. We'll see what happens this time but one thing is for sure if it isn't right this time I won't be buying another HTC mobile or recommending it to anybody ever!