BB Curve :: It Turns Off And Resets For No Reason?

I have a blackberry curve 9300 which I bought in January this year and from practically the first day I turned it on I've had problems with it. The phone freezes a lot and on a regular basis the phone just turns off and resist itself. I can be texting and look away and look back and the phoneheads just reset and it does this a couple of times day. Less than a month ago it was sent for a repair with Tesco (who I bought the phone from) and they sent it back saying that they couldn't find a fault on it. The phone still does the same and I've phoned tesco again and they have said that the only thing they can do is to send the phone to the repair shop again to be checked. What do you suggest I do as I need a phone that works and cannot just keep sending it to be repaired to be told there isn't a fault over and over again.