BIOS makes random changes to setup

For several months my BIOS has made random changes primarily to boot order. IAt first I thought it may be a USB legacy thing as it tried to boot from my external HDD. It wasn't often and I just entered setup and put it right when necessary. I checked the battery and it was 3V+.

Recently it got worse and started not only changing boot order but also storage config from RAID to SATA.
Again it's random, not every day, not the same changes.

I replaced the battery with a brand new one 3 weeks ago and it was fine for a fortnight then it did it again today.

My wife bought a new iMac last week (WHAT I hear you say!) which is connected to our workgroup and reads the PC shared files BUT the shares disappear because despite being on the same internet time server, the PC clock is a second or so ahead. Perhaps down to BIOS? When the clocks are out of sync, the shares disappear.

I doubt the CMOS battery has run below 3V in just 3 weeks, so I'm guessing there is more going on.

Any thoughts welcome

Did you try resetting CMOS? Or load defaults in BIOS?
1 second shouldn't cause a problem in networking.

I haven't tried resetting or loading defaults. The PC is 7 years old and I was planning to strip it, give a good clean and check all the PSU voltages when time permits.

What effect does load defaults have? Does it give a clean staring point?
The USB external is always attached and is mapped as a network drive.

The shares disappearing and re-appearing is really odd. Googling suggested that Win 7 was really tight on time and sharing. It could of course be a Mac thing. I know nothing about them!
Thanks
Pete

It will make the BIOS forget settings and what not and turn back to factory defaults. You will have to redo boot order and things like SATA mode back to RAID. If you know your way around BIOS (which seems you do) it is not a big issue.

Network drive is an OS configuration. BIOS reset won't affect it.

I was just looking through all the BIOS settings. Most are probably defaults.
Other tha boot order and RAID, there is nothing fancy so I could load defaults and renew settings with ease.
Ironically the BIOS system clock is spot on with the Mac.
Core & system 5V & 3V voltages are fine. 12v is a bit low at 11.5 to 11.9V. Temperatures are normal.
I'll wait until it plays up again and load defaults while I'm in there
Could it just be MB old age?
Thanks
Pete

In a case like this I normally reflash the bios, to an upgrade if available, but even the same version if that's all there is. Power failures, surges, etc can mess up the bios. Of course, there are risks in any bios reflash.

It's probably a good idea. BIOS is the latest version but it's about 4 years old.
I'll do that as part of my full service.
We have had a number of power outs over the years so it may hay have affected it.