It could not be the battery. Unless there are SERIOUS faults in the lithium cells, it will not just shut off like that. The EEPROM is very significant. EEPROM (Electronically Erasable Programmable Read Only Memory) is usually used in your CMOS. There was some sort of error and it reverted itself back to the previous flash. Please download SpeedFan and report back with your computer's temps.
http://www.majorgeeks.com/download337.html
The checksum error indicates a bad CMOS battery. If you left it without charging for a few months, that will cause the RTC battery to drain as well.
I would try replacing the RTC battery. If I get to it, I will try to do some research on that. If not, sorry.
__________________
Repair Windows XP Guide ---
Resolutions
Crazy Computer that I'm saving up for::Antec P180 Case;PC Pwr&Cooling 1kW Quad SLI PSU;Asus P5N32-SLI SE Deluxe;Intel Core2 XE X6800;Corsair TWIN2XP2048 2GB PC2-6400 DDR2 RAM;2xAsus GeForce 7950 GX2;BFG PhysX Accel.;IBM DVD-RAM/Plextor DVD±RW;Seagate 750GB HD 7200RPM;WDX 150GB 10000RPM;Vigor Monsoon TEC Cooler