OK. I think I get it. Its definitely the battery. I think the reboot is cause by the WiFi draining the already weak charge battery too quick for the ac power to recharge it fast enough to keep the device on, and the device does not turn on without the battery. So It's not actually rebooting. The device actually turns off while the ac power turns it back on, as it always do, to charge it. I'll be buying a new battery, soon.