When you say "screen goes black," does the computer power down, or is it still operating? Does the sleep light come on? Does it respond to the keyboard and mouse? (Example: press the power button momentarily, wait a few seconds for the "Do you really want to shut down" message to appear, even though you can't see it, then hit the return key to answer "OK," and see if it shuts down.) Does the screen go black or come out of black if you tap the computer?
While your computer is working, go to System Preferences / Desktop & Screen Saver, the Screen Saver tab, click Hot Corners, and set a corner of your screen to Put Display to Sleep. Then next time your screen goes black, mouse your way into that corner, pause a few seconds, then mouse back out and see if your screen returns.
One other test: get an external display and hook it up to the iMac through the display port. When your main display goes out, does the other display also go out? This is probably the diagnostic that will tell you the most about where your problem is. My first suspect is a bad inverter or backlight, which won't affect the other display at all. But if both go out, then you're likely looking at a graphic processor or logic board problem, which is expensive.
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