I had problems with the irql_not_less_or_equal for about a year or so. Everywhere I looked everyone said it was caused by either driver conflicts or bad memory. However in your case, if CPUZ still says the clock speed of your processor is 620 mhz then that is most likely the problem. I would defiently recommend reinstalling all of your drivers again without having a virus scan program running. Install the chipset drivers first then your graphic card drivers, and then anything else that you might need to install. Also, if you are able to update your bios cuz that might help. I know with me it did atleast. Good Luck!