|I believe that on many systems you have to have AHCI enabled when you install Windows, changing it later is not as easy. If I remember correctly, there is an article about doing it on Microsoft's site with the details.|
It does begin to sound like your power supply is the problem so If you have another one you can substitute in, it will help you be able to tell.
Boot into BIOS and check your temperatures there and check your actual voltages there. The temps must be reasonable and your voltages must be within 5% of their nominal values (ie: 12Volts +/- 5%, etc.). Post results.
The temps and voltages may help you/us decide where the problem is.
Boot into Safe Mode and download your drivers again from each component manufacture's site and install them.
If you do not get freezing in BIOS and Safe Mode (assuming that temps and volts are within range), it is very possible that it is driver or software related. Of course, it the problem only comes up under high video load, it may still be power supply related.
Install HWMonitor to give you are real time reading on these from within Windows to see how things change over time and with increased load. Report numbers at idle, 10 minutes of normal use, and after 10 minutes of gaming.
You have to be a little bit crazy to keep you from going insane.