I had a similar problem a while ago, Mitsubishi CRT went dark - sent for repair.
Tried an aged LCD monitor and all was fine, occasionally the LCD would flicker, but not being familiar with LCD's I thought it was resynching or something, worked ok for 2 months, then it went phut!
Twas the graphic card after all.
I think the LCD was more tolerant of the graphic card fault than the CRT (which couldn't be fixed due parts no longer made).
Now on new mobo with Sapphire Technology Radeon X1900XTX 512MB and Dell 24" widescreen - nice.