computer screen goes black but computer still running

I was told the hardware should be perfectly stable. Next, the iMac returned to an 'Apple Authorised Service Provider' for some heavy and prolonged stress testing, but again no hardware errors were found. All hardware has been extensively tested with the 'Apple Hardware Test', but with no abnormalities found. I suspected a hardware related problem in the beginning.

I've been experiencing the above described iMac problems for some time now. I should mention that I have 3.06 24 with Nvidia - other threads suggest that on the new 27 models the graphics drivers may be at fault, but that's with ATI - I have black screen, with Nvidia. Now it's black again and I can't snap it out of it. Yesterday I turned it on the store, before boxing it and taking it home again - it was fine in the store, and fine when I got it home - for an hour. The really annoying thing is that I had black screen when showing them in the store, then they boxed it and took it upstairs to fix it, and since they unboxed it in the repair room it behaved completely normally and passed all tests with flying colours. He put everything back to 'stock' before I took it to the apple store. I've also had this mac with an apple repair guy for a few weeks - replaced 2 screens, replaced screen power cable, replaced system power box, all to no avail. Black,black,black - but the computer is working perfectly, apart from the fans sound a little different when waking from sleep or starting up. Then another, then another, and since the third one, I haven't had the screen back since. They'd had it there for a week and the 'Geniuses' couldn't replicated the problem. Got my Early 2008 24' iMac back from the apple store yesterday.

Computer screen goes black but computer still running