iPad switches off randomly.

I'm sure this topic has been covered countless times before but I'd just like a quick answer from those in the know! I have an iPad 2 16gb Wifi. Just bought a Griffin leather case for it. Ever since I've used the case the iPad will random switch off and go into the lock screen. When I punch my code in it returns to where I left off. I've heard that it could be something to do with magnets in the case, but feeling round I don't think there are any - certainly non that are obvious and hold the cover closed.

Any advice will be welcome.