FFXI Software Issue?

Hello, i am in need of great help from a computer genius. Recently my computer has been turning itself off while running FFXI(Complete black screen and doesnt boot back up). Now you might be thinking off that start that it is a over heating problem but it is not, i have been playing XIV for a few days now on default graphics and have had no problems. While playing XI my GPU temp is around 54-58c and on XIV its around 65-77c. I also just went through the lovely 8~ hours of reinstalling XI to see if maybe there were some corrupt files and my PC is still turning itself off. So if anyone has any idea or is having this similar issue your help will be greatly appreciated!

System specs - Intel Core2 Quad Q8200@2.33 Ghz
ram - 8.00 GB
64 bit windows 7 professional
750w corsair psu gpu - gtx260

I know that could be a possibility and i would hope its neither but i dont understand why i cant run a 9 year old game for more then an hour but i can run a game thats been out for 6 days and that requires way more system resources...

I forgot to mention i did try the factory PSU and still have recieved the random shutdowns. Also i have no problems at all starting the computer back up.