You already mentioned changing the battery - that is always the best place to start
If you have Any USB powered devices (with separate power to them) disconnect them - they can back-feed the MB
Use your phones camera & capture All BIOS Screens to save your settings
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"The BIOS Dance" - as I like to call it
these steps have helped on Many an older MB
After removing all externally power devices - set BIOS to default, save & reboot - then - load optimum BIOS, save & reboot
Now remove All power pull the plug to PC & monitor. Hold start button 30 sec to discharge Any capacitors in PSU & MB
Plug in Just PC & Monitor - No external powered devices - Open Bios & place Your previously saved settings, save & reboot
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I'm leaning towards the CPU is tired ... but yes BIOS corruption can happen ... the question would be why ?
If the CMOS is going south (limited number of writes) trying to Flash might not end well - especially if this is you only PC
I would Never flash a BIOS without using a UPS to make sure the power does not get interrupted mid Flash