Memtest is nice, but you've got to run it for hours to get a real test. Most of the time people don't run it long enough, and the returned value are ok, when in fact the memory module is deffective.
Too true that. The more memory you have, the longer it takes as well. It's still a good tool though so long as you've got the patience, especially if you're testing overclocking stability where you're not just tweaking frequencies but timings as well. Errors due to bad timing choices usually show up in the first pass. It's been my experience that what's written on the label isn't always 100% accurate and incorrect timings can certainly cause the symptoms described.
Speaking of overclocking, that's another thing Monkeybrain may want to consider, especially when it comes to newer hardware like he has (e.g. speedstep). Pushing the CPU and motherboard too far is another thing that can cause the symptoms described, which if I've understood correctly involves his PC automatically turning off approximately 3 to 4 seconds after turning it on, as well as occasional freezing and checksum errors during post.
My best guess is that it's either a BIOS related setting(s) or faulty hardware to blame. Hopefully it's the former. Seeing as there are only so many possibilities for this specific type of issue, I figured posting as many as I can think of might be helpful. What would be really great is photos of all his BIOS settings, which would go a long way towards narrowing things down for him. The only thing that can be ruled out for sure at this point is the operating system. I'm sure if we all put our heads together, we'll be able to get to the bottom of this problem in no time at all.