I've got a mid 2012 MBP with an i7. 750gb HDD has been upgraded with a 256gb SSD, it is super fast but freezes and shuts off whenever you move it or type too hard on the keyboard. Purple/multicoloured squares also appear on the screen when this happens.. Not sure it's going to be lasting much longer...
This sound like the Apple's always returning GPU soldering problem: https://www.apple.com/support/macbookpro-videoissues/