motherboard or ram?

I bought a used motherboard,cpu,ram. M5A78l-MX-V2[latest bios], FX6300, 2x4GB Mushkin.
It came up saying I had around 4GB in 64bit Linux. I assumed he had made a mistake about the ram and it was
2x2GB; I hadn't had a look at the ram before I stuck the board in the case and I have a bit of a whopper of a case
which is stuffed in the corner[pain to drag it out to look].

I complained about the ram and he assured me it was 8gb, "run memtest".
The bios said 4096MB and it posted with 4096, but memest had: "SPD information": 2 pieces of 4096MB but
memory available: 4078MB, and it only ran to 4078MB in it's tests!

Then I was using it a few days later and noticed I had 7.8GB of ram available. Bit more fiddling and it seems if I
watch the post screen and hit ctrl, alt delete if it says 4092 it will eventually say 8192, then linux will say I have
7.8GB of ram and memtest will test it all. I tried reseating each stick and swapping them over, made no difference.

Anyone any idea of what it could be?

What speed is the memory supposed to be?

What voltage is the memory supposed to be?

Have you tried testing each stick individually, to see if one of the sticks in particular is having trouble being detected?

I'd also consider setting the BIOS to "failsafe defaults" or resetting it completely.

Just make a note of any disk setups you may have i.e. RAID, AHCI etc.. as the OS may not boot.

Reset the cmos, set it to defaults, memory picks the settings on the sticks.
Nothing to do with OS. Once you have got 8gigs in post it will mem test and run an operating system with 8gigs.
If you let it go past post with 4 gigs showing in post, memtest will show 2 sticks of 4 gig, but will show 4gigs of memory and
only test 4 gigs. And linux will only show 3.whatever[shows 7.8 when 8 gig shows in post].
I have tested both sticks in each slot individually and together.
Not tried changing voltages? Will have to look at memory and see what voltage it's running at.

Have had ram in the past that was the older 1.65V standard which works fine in AMD motherboards *if* the bios gets the voltage right but for me it didn't. At 1.5V odd stuff like this happened.

Do you have another machine you can swap memory around with?