So as the title says I have a 27" iMac 2009 that only boots to black, if I reset the PRAM then it boots fine and runs fine all day! Any suggestions? I've googled to find lots of similar problems but mostly to do with failing video cards, as the machine runs fine I don't think it's the card, it seems like the battery has gone on the motherboard so it's not retaining the resolution. Only other thing is it could be a RAM thing, it's got 16 MB of ram but the machine passes all the hardware tests including the official Apple service test I managed to get hold of. Appreciate and feedback I'd really like to sort this thing out, the only other thing I thought of was upgrading the video card, I've seen some Radeon HD 6770's on Ebay.