I recently had this problem (as in last week) so I figured I'd post it here from my laptop.

I was playing CoH when my Pc suddenly went black and hung, so I rebooted. When I did, the screen was garbled and unreadable, resetting to a NO SIGNAL, then the corrupted screen again.

I've tried letting it cool, thinking it was overheating (which cured it for a week when it was running from 55 C to 71C), I'm unable to actually get inside the case to check the card itself due to the side of the case being hooked onto the rest of the case.

I tried the monitor on another computer and know it works fine there.

quite likely it is broken. a case being hooked onto the rest of the case? unhook it....

i have a nice 8600gt for sale if you need