This is pretty similar to what is happening to my mac. I downloaded the Yosemite update and it took forever for it to download. Once it was up and running it changed a lot to my computer. Once I tried to edit my photos they new photo system said my photos were corrupt and then everything was corrupt on my mac... then the screen kept going black and I have to keep unplugging and plugging back in the computer for it to turn back on... We took it to the apple store right away because i'm a photographer and I NEED my mac for editing and such. The technician ran tests and said my hard drive was bad. So I payed for a new hard drive, got my computer back, took it home and the screen would not turn on and it sounded like a race car. I took it back the same day, they said they forgot to plug the wire that connects the screen. Okay, they fixed that and I got to take it back home the same exact day. Turned it on and started editing my photos and then the screen keeps going black and i'm so frustrated. So its not the hard drive. Made and appointment the next day to have my computer seen AGAIN. He ran more tests and bla bla bla kept for a couple days and called we are just wiping your computer, restarting it and this should fix the problem. I take it home, turn it on start working on my photos and BOOM THE SCREEN WENT BLACK AGAIN!!!!!!!! I am a total loss, what the could this be, my computer is only 2 years old, my husband purchased it for me in 2013... Our situation sounded the same and I was wondering if you knew what exactly they said was wrong with your computer. I feel like YOSEMITE had RUINED my computer.Please and Thank you.