
Most likely reason is that your backup battery is failing - this battery 
maintains the contents of the CMOS memory when AC power is turned off, and
if the battery is flakey then the contents of the CMOS will be lost and 
the checksum will be wrong (along with most other of the CMOS data). Try 
replacing the battery.

If, however, your PC doesn't use a battery but a large capacitor to power
the CMOS, you should check to see if you can replace the capacitor with a 
more normal lithium battery. If this isn't possible, make sure you leave
your PC on for a half hour or hour each day; this will keep the capacitor 
charged. I'd opt for the battery change, though.