Recurrent Fatal System Error at start up

Hello, I built a new PC in October last year (first time build), and it worked happily enough until December when I started getting error messages at start-up. I made a note of the first one which was:

"STOP c000021a {fatal system error}

The initial session process or system process terminated unexpectedly with a status of (0xc0000221 0x001003f0)

The system has been shut down"

After some constant rebooting and battering of the F8 key I was able to get it to do the 'revert to an earlier time' thing, which fixes things for a while, but the error keeps coming back.

When the problem strikes (which is pretty much every 1 or 2 days now) the messages seem to be slightly different each time; most recently it was 'paged fault in non-paged area', for example. In all cases the only way to 'fix' it is to hit F8 and keep doing system-repair or 'revert to last known good config' repeatedly until it eventually decides to start working again.

Memtest86+ came up clean after 8-10 hours or so, and the memory timings are reported as:

"RAM: 666MHz (DDR1333) / CAS 9-9-9-24 / DDR3 (64bits)"

...with CPUID confirming this, and listing the voltage as 1.50v, which (along with the previous settings) is what it says on the sticks of RAM in my machine.

About a month ago I finally had enough of trying to troubleshoot software/driver problems and did a full Windows 7 reinstall, and set it up to dual-boot with Ubuntu. I'm still having the same problem, and when the problem strikes it won't load either OS, which I'm assuming rules out a software problem.

I finally hit upon the idea of waiting until the computer wouldn't start and then trying memtest a few weeks ago, since it's one of the options on my bootscreen now. Lo and behold it reported a bunch of errors, literally within the first 10 minutes. 7000 or so on my first try, then over 10000 on my second.

If I try memtest when it's 'working' it passes fine.

So, I've no idea if that points to bad memory, or whether the fact that it's a really high number of errors, produced only when the computer won't boot suggests something else?

Failsafe settings in the BIOS makes no difference. I've disabled 'spread spectrum', too.

Once it loads up, the machine works fine, for hours at a time if necessary. (The only thing it does once booted which might seem 'unusual' is to make a LOT of noise if I run games, but I've read that the stock AMD cooler is very noisy anyway. It does sound a bit like a washing-machine under heavy load but I'm guessing that's just normal until I get round to buying a better cooler and probably unrelated. It's fairly quiet under normal usage).

System specs are:

Motherboad: MSI 880GMA-E45
CPU: Phenom II 955 BE
Memory: 2 sticks of Corsair Dominator DDR3 1333
GPU: Gigabyte GTX460 1GB
Hard Drive: Samsung SpinPoint F3 7200 SATA
DVD-ROM: Samsung SH-D163C 16x DVD ROM 48x CD ROM
PSU: Antec 500 Green
Case: Coolermaster Elite 340

I'm completely out of ideas on this now. (It was Sept last year I bought all the stuff to build the PC, so obviously I don't want to start harassing Scan for replacement RAM unless there's a good chance that's actually what's causing it).

Can anyone shed any light on this, or suggest some further troubleshooting steps to take?

Thanks for reading.