IMAC 27" SCREEN GOES BLACK

The screen on my iMac lights up upon startup however about 2 seconds later goes dark. It sounds like the hard drive is running but no screen. Occassionally, (ever 30th try) it may go through the entire startup but then shut down a minute or two later. The condition has been getting progressively worse over the past 3 months. Now it is very hard to get the screen to light up beyond the 2 seconds.