Chaz, I've been having very similar problems as what you've mentioned and also taken the same troubleshooting steps (maybe not as extensively) as you to no avail; minus reinstalling the OS / swapping to a different OS.
I have Windows 7 x64 (Ultimate) as well and have random crashes/freezes. Luckily for me it's not restricted to gaming and will happen at random, even while just browsing or watching Netflix. I've even had it freeze up while completely idle and not in use! It's so annoying since I let my computer sleep and rarely shut it down and all the site login cookies become corrupt and I have to login to everything all over again. I've done this about a hundred times now and it really pisses me off, lol. A good test of my patient and I'm afraid to say it's winning.
I've run memtest86 for several hours, but never more than four. I'll try this while I'm at work tomorrow. I'll admit that 4 hours (even 4 instances utilizing all memory and nearly 100% of CPU) won't be enough to produce results.
Regarding your thoughts on the latest generations of graphics cards being the culprit, I can say it's not the case - for me at least. I've experienced complete freezing now with X800, 7950GT, HD4830, and HD6870.
The only commonality between our builds other than OS is our MoBo brand, Gigabyte. /sideeye
I really hate wiping, but at this point I'm thinking that it's my only option. :/
Edit - Stupid question: What do you mean by TDR? Timeout Detection and Recovery?