computer restarts

Just built a computer p4 with fic 1st mainboard.
It has 2700 ddr 512 mb ram. It calls for 2100. The system works ok except it reboots when we shut system down. It will also stall for long periods of time.It continues to reboot and stall. It only shuts down in hibernation mode. Does anyone know if this is hardware or software? We are using xp.