Peanat

Good day I have a question for a Technical hand. Or anyone else that can assist me. I have a very unique fault on my pc and seem to not find the solution. My computer starts up all fine and I monitored my temperature on various parts in my computer and all is at normal with the highest at 50 degrees C. My computer will just randomly do a "warm shut down". Meant by that: The screen will go blue. Sound will stop working and so will everything else. Here is what I have never encountered before. All my fans in my PC will continue to run at full speed. The only control I have over it is opening of the dvd rom and force shut down, by holding the power button in. Thats it. Nothing else. When I turn it on again it would run fine. Sometimes it does not even do this at all and sometimes it does it all the time. My gut says to my it might be my power supply. I tested the power supply at a local shop and the tested it all fine. There is no pattern to follow the fault which makes it not so easy to me. Please assist me and feel free to ask any details. I am in dire need of some expertise. Thank you Asrock G41M-VS3 R2.0 Socket LGA775 DDR3 Mainboard Intel® Core™2 Quad Processor Q6600 (8M Cache, 2.40 GHz, 1066 MHz FSB) Axtrom 2x4GB DDR3 1333MHz 3TB Seagate Hard drive 7200rpm Asus EN GTS250 1GB DDR3 1Gb DDR3 Nvidia Graphics Dany 700W Power Supply PM70 ATX