hmm with the same settings and clock speed, try to boot up into safe mode several times, if it boots normal all of the time, most likely it is problem with some program loading up in normal windows.
if it has same problem in safe mode, try to clock it back to 633 and see if that helps. if it works ok, its problem with 633 not being cooled enough or not enough juice, clean the mess up and try arctic silver and maybe up the voltage a little.