PC reboot by itself

I’ve this pc which is running Win98 always reboot by itself several times every morning. The pc will keep on rebooting until it is able to boot up in safe mode. If you restart the pc again from safe mode, then Win98 is able to boot up. Subsequent later, if you try to restart the pc again, it will restart without giving you any problem.

The problem occur only when the pc “rest” for too long.

I’ve changed the VGA card, LAN card and even update the latest bios but still doesn’t help much.Pls advise.