Possibly a power problem. You need to check the voltage at the EPROM power pins and make sure it is 5.2v. If it is not adjust the power supply so you have 5.2v at the EPROM (Top right+, bottom left-). Also could be AC ripple on power supply. If it does prove to be power possibly you could configure a UPS (Uninterruptable Power Supply Battery back up) to keep power on the board when glitches occur. Power companies do a lot of strange things at off hours.