Ok I bought a 15" MBP from a local college kid and it is giving me fits. It had a 500gb HD and 8gb memory running a i7 2ghz processor. Of course I was told it had no issues it looks in perfect shape. It seemed really slow after boot up and HD when upgrading to yosemite had SMART errors so I ordered a Samsung PRO 250 SSD and 16gb of mac certified RAM. I of course have a clean install of Yosemite. Ok system boots up like lightning fast. And it runs fast for lets say 5 minutes then almost becomes unusable. Activity monitor shows CPU Idle less than 10% And thats after I quit all applications its remaining that way. I reset PRAM several times still no luck. Any suggestions appreciated where the issue may be.