iPhone keeps rebooting 3G

Hi all
I cracked the screen on my iPhone, took it into the Apple Store and had it replaced with a new one for £130. Ever since Ive had this new iPhone, Ive had problems with it rebooting. A lot.
Its absolutely fine while in standby or while on a call. If I use the Internet, use Google Maps or use the camera for more than a couple of minutes at a time, the screen goes black and then the Apple logo appears. The phone then reboots repeatedly until I switch the phone off completely, leave it for a while and then try to start it up again.
Ive tried everything, including a Restore via iTunes, but the problem persists. Ive got time booked in at the Apple Store tomorrow but wanted to know if anyone had any suggestions on how to get this fixed myself?
Thanks
iPhone 3G 8GB iPhone OS 3.1.2