operating system not found on any device

Hi,
My client is receiving this error message after the computer is on for a while(1-2 hours) and it reboots.After it reboots the message operating system not found on any devices press any key appears. The system has run for as long as a day without shutting down and then on a manual reboot the message appears. I have tested the drive with spinrite and one sector at the front of the hdd was recovered. When I change the cmos battery right after the message appears it will boot test RAM amount give 1621 system options not set bios updated and 163 system time error and boot.