Trying to get a second monitor to work

[bodingtonsboy]

Hi there iam tryin to install a second monitor (sony kdl-26U300) this has been connected via a 5M HDMI to DVI CABLE w/FERRITE the resolution is set to 1366*768 and 60htz rr. When this is the secondary display (as an extented desktop), it seems to be ok but when i make it the primary one(which is what i need it to do) after a few mins the display starts to fade and go blocky then freezes altogether. There is no warning from my graphics card to say it is overheating so i dont know what i'm doing wrong, as you can imagine im very concerned for both my monitors and my graphics card, what do you recommend ?