I had this problem with mine, fortunately I was still within my warranty period while it started to play up. It was temperamental though, sometimes it would work fine and other times it wouldn't.
I had a rather different experience with the Apple store at the trafford centre, they wouldn't replace my iP4 as was working fine in their hands, even though the guy failed to understand that it was an intermittent problem. I had even recorded a video of the problem which at no point he was interested in viewing. Anyway, the guy restored iOS and said it was the software that was causing the issue. I was calm about the whole situation but I found him rather condescending.
Anyway, left the store and it started playing up again - no surprise, couldn't be arsed dealing with them after that, didn't want to kick up a fuss in a store full of people. My first experience at an apple store wasn't a particularly good one I'm afraid, I'm just hoping that's a one off but it has put me off going there again.
Anyway with a bit of luck you may generate some success in getting a free OOW repair or a replacement phone. I think who you deal with has quite a lot to do with it to be honest.