I have had the same problem with my 2009 iMac 27 inch as well. It worked fine for years, and under Apple Hardware Replacement, Apple offered me a new Hard Drive due to issues. this was after 2 10 months of ownership. I took it to Apple, and they replaced the Hard Drive for me. To my surprise, they said my iMac had some issues with video, and replaced my main logic board, and power supply as well. When I got my iMac back, my fans seemed to run at a higher speed, and I called Apple on this, they said sometimes they will, and if I wanted, to take it back to the Apple Store where the repairs were done. Since this is over an hour drive for me, I passed on this option. About 6 months after the replacement, I began having black outs on my total screen as well. By lowering the brightness, I was able to control the frequency of these blacked out screens, but it is to the point now where I am only able to use my iMac with either one or sometimes 2 bars of brightness level. I checked online for issues, and found a couple ideas, then opened my iMac. First thing I noticed was that the 4 prong cable into my inverter board was not quite seated properly, as one of the 4 copper tabs was folded over. Upon looking at the inverter board where this cable slides into, I also noticed one of the capacitors had blackened areas at the contacts. I purchased a new cable, and inverter board as well, and installed them. This seemed to help for a few months, but now I am back to blackened out screens constantly, and using only one or 2 bars of brightness. After all these replacements, the only items I can think of is either the video card, or the screen backlight itself. Will investigate more, and hopefully get back. I have worked on Macs since the Mac Plus, and have some circuitry experience, so I'm prone to believe that each instance is a bit different, as each user has different loads he/she places on their computers. Also, having printed circuitry for years, I am aware of failure rates on boards, and different circumstances that can cause them to be accelerated. Bottom Line, small tight enclosures will have some issues with heat causing circuits to become faulty. The particulars are far too many to pinpoint except from the circuit printers' testing sites, and again, may be overlooked with fault rate parameters set for production. We all have the issues, but many things may have caused them. Personally, I love my old iMac, and will pay to get it repaired and possibly, even upgraded, if even by myself doing the work.
