Xbox 360 restarting automatically?

Has anyone seen this happen and know of a solution: In the middle of a game or while on xbox live the machine will automatically restart? There are no error codes, or red rings just a restart/reboot. The machine is well vented (on an open shelf). This has happened while playing lord of the rings conquest both while playing standalone and connect to xbox live. It happens randomly. Machine is brand new - take out of thebox a couple of days ago. There is no hard drive and there looks to be about 78M of memory left. Thanks