OK, I've solved the problem. Let me just say - with no insult to your abilities - that it's lucky I figured it out, because you would NEVER have guessed it.
It wasn't a problem with the RAM.
It wasn't a problem with the Timings.
It wasn't a problem with the Frequency.
It wasn't even a problem with the motherboard.
Now that I've wound you up....it was the power connector on HDD #3!!!
The little plastic "L" end on the SATA power connector seems to have broken off, shoddy piece of plastic crap. I know it's that HDD because it would boot with everything installed except that one. I don't understand why it's a problem, cause all the terminals seem to connect fine, which they obvious are now that it's up and working again. Something must have been a short circuit in the PSU or something, so I have not idea why the motherboard chose to hang on the RAM check every time.
How did I fix it? I jiggled it and pushed it up a bit.
Why did it go wrong in the first place?
I must have pushed a SATA power cable when I was reinstalling the VGA card, which either broke the connector or moved some terminals on the already-broken connector away from what they should be touching.Why did it work briefly then stop again?
In my fiddlings in the case trying to find a cause of the problem, I must have accidentally pushed the connector back to where it should be.Why did it then stop?
When I went to change the fan connector over, I must have bumped it again!!
I hope this helps anyone who ever has such a bizarre problem. And thanks to everyone who offered a possible solution, especially you, Sharp, your idea got me to figure out what it was.
What a crazy, stupid problem! Still doesn't tell me why it's so bad at overclocking, but maybe I will find that out when I borrow my friend's RAM to test on my machine.