I have a 2011 MacBook Pro i7 and I have upgraded the memory to 8 GB and the HD to an SSD 500GB. The other day I had it running for 3 days non-stop and when I tried to use it it was frozen. I reset it and when it went to the boot screen the graphics looked funny like 16bit color mode, and it got stuck after a couple of seconds. I removed the hard drive and I noticed that I can access all the files and I backed them up. I downloaded Yosemite and I tried to install it from an USB stick but after loading for a while the screen turned gray and it froze. I'm a bit baffled as what might be the problem? Do you think it's the hardrive or is the hardware failing? I'm attaching screenshots of the bootup srceens.