Using your Digital Volt Meter, what voltage is that CMOS battery reading?
The other clue is the trials of other BIOS versions. That's never a good sign. I think in 20 years that paid off 2 times. So go back to the ORIGINAL BIOS issue.
Sadly, as told it looks like a new mainboard is needed.
A standard boot up will not happen. This laptop will only boot up if you take the cmos battery out for 20 minutes (and the mains power and the main battery) - ie BIOS memory cleared.
If I choose restart from Windows it will not boot - there is no Bios info or options - the power light comes on, the HDD LED flashes once and the fan spins for 0.5 seconds. Then it tries the above again.
I have loaded up new versions of the bios and tried all kinds of chk disk and googled for hours but seems all advice is just to replace the CMOS battery which I have done already....
I have re-seated the RAM, used one then the other, in both RAM ports, doesn't appear to be a RAM issue.
Any ideas? or is the mobo toasted?
PS running Windows Vista Home Premium which boots happily from a cleared Bios memory.
PPS Exact model number is HP G6065EA