System not booting with a CMOS error

Ok, got an older system running Windows 10 which has decided to act up over the weekend. No changes had been made to the system.

Power on the system (system is not used that often, maybe twice a month at most) and about once in 10 times it gives a message about CMOS being corrupt and prompting to use a previous version but the system doesn't really get any further than than what ever is chosen. The other 9 times the system sits there spinning fans but has no other response.

Occasionally it's possible to get into the BIOS but that didn't really show anything is wrong. Note that once the system has been able to get this far it then has to be powered off and disconnected from the wall for a while otherwise any boot attempts will not produce anything (other than spinning the fans).

Tests are performed with drives disconnected.

The system has:
mATX [FONT=Verdana, Arial, Helvetica, sans-serif]Gigabyte GA-MA78GM-S2H [/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]AMD Athlon 64 X2 4450e
4x 1GB DIMMs

With an 128GB SSD, 1TB WD Blue HD and an optical drive. The motherboard has sound and video onboard so no expansion cards are fitted.

I have tried clearing the CMOS and replaced the onboard battery with no improvement.

The next thing I'll be trying is removing most of the memory and swapping through it one at a time in case a DIMM has gone bad.

Any thoughts?

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I would take all your memory out and leave just one module in so there are no conflicts, as i have had this happen before when installing RAM and it took a while to get them working together. Can you get into BIOS at all?

The times when it actually starts enough to give a CMOS error I can get into the BIOS if I tell it not to try and load any previous configurations and to just continue. I have tried loading defaults in there which made no difference.

I will be trying the memory as you say at the weekend.

I would also note I plugged in a PSU tester today and it was showing the 12V reading as being 11.7V which I don't know is a bit low considering the lack of load on the PSU when it's only hooked up to the tester.