Because that power supply has probably degraded over time quicker than a better brand supply. All power supplies will fail after a lot of use. Corsair, Antec, Thermaltake 650 or so watts. Here's an example:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371044
Hi there Tmagic650
I have an i7-2600k system here, and I´ve had 3 of these shutdowns (bluescreens)... and cant find anything wrong here...
Have an Corsair 850W http://www.corsair.com/tx850w.html
Everything shows allright when it comes to voltage and amps... do you have any other suggestions ?
Thanks in advance
Kenneth