System clock loses time (new CMOS battery used)

I built my PC nine years old. During the past nine years, the system
clock ever suffered time loss twice. I changed the CMOS battery each
time , and the clock was OK again.

But this time I can't get the clock back to normal by changing CMOS
battery.

I booted to the CMOS setup screen. Adujsted the system time correctly.
Saved the new settings. Then turned off the PC. I did not boot into
Windows or DOS. (I use Win98 SE.)

Thirteen hours later, I turned on the PC again, booted to CMOS setup
screen (not starting Windows or DOS). I noticed the system time has
only progressed for forty minutes!

The system clock has gone slow during the last 20 days. I've tried to
survive by automatically adujsting time through a NTP program.
Yesterday I replaced the CMOS battery with a new one at last. But
the problem persisted.

Whatelse could go wrong?

There must be nothing to do with Windows or DOS because I did not boot
to either OS.

The APM has been disabled in BIOS since the day my PC was built.

The mobo was PCChips M575S