I have only even run memtest on individual sticks **after** failing with all sticks installed.....since you passed 10 hours with the kit, albeit with a run a little shorter than I'd like, I'm guessing problem lies elsewhere.
If you are freezing up, you should have the errors defined in event viewer. What are the Event Viewer messages ? Are mini dump files being created (look for *.dmp files on C).
Turn off the extreme settings and see if it occurs..... if not, your overclock isn't cutting it. 4 RAM modules can be a bit too much to handle on these OC settings. I have oft had problems with 4 modules that require reducing timings in order to maintain OC's that I could easily handle w/ two. But again, these problems popped up running a 12 hour memtest and you were able to pass 10 hours.
Still, I'd try w/o the extreme settings. If that solves the problem, do a manual OC instead of the preset. On a side note, you may want to consider Mushkin or Corsair next time around. I can't seem to get the same stability out of Gskill as I do w/ the other two.