I'd first recommend clearing the event log and loading the BIOS default settings:
Restart the system into BIOS by pressing <F2> during the boot process. Once in the BIOS setup utility, press <F9> to restore BIOS default settings. Reset any customized BIOS settings. Clear all DMI event logs by setting the option to "Enable", which is located in the Advanced/Event Log Configuration section of the BIOS Setup utility. Press <F10> to save the new settings and reboot the system.
If the problem continues, although the system had worked fine before, I'd suggest ensuring your system's using officially, tested memory: