Long story short: -MBP 13" mid-2012 2.9Ghz, at a later stage (around mid/late-2013) upgraded with 16Gb ram, 256Gb SSD (samsung 840 pro), and optibay with original 750Gb HDD. -mid january 2015 I come back home after 3 weeks holiday to find my MBP (that had remained home) not working properly: fan runs crazily fast, system runs crazily slow. Further investigation lead me to notice "kernel_task" taking about 300% of CPU, and apple hardware test returns me a "4SNS/1/C 0000008:TAOP--124 " error. -I try a little bit of everything including restoring original optical drive, and testing my SSD externally on another MBP (working fine), or another disk with fresh install of the system mounted externally. I then send it for checks in a apple authorised repair (please note I live on a small and remote island 300kms away from first apple support, so it's not like I can take it anytime and within minutes or hours), but I keep my SSD home for privacy and safety of my data. They call me to ask what is wrong, because they can't find any problem (even after a 24h hardware test).. and eventually blame my SSD that I hadn't sent (but I know it works, at least externally, on other computer). -The computer comes back, I mount my SSD externally and it works. I mount it internally.. and back to usual problem. Mount it externally again.. still the problem (so it seems not to be the internal cable). - I decide to order a new SSD anyway (I might recycle one on my PC anyway), so I order a samsung 850 pro this time.. and well, even with brand new disk (although I had done, through another computer, a copy bit to bit from the old one) same problem. It's about mid february and a few days later I'm going to Milan.. so I decide to take it to apple there... but when I arrive there I talk quickly with the guy at the desk and he asks me to show him the problem... so I turn it on and... magic: it works! It was the 20th of February. -From 20th of February till 17th of March it worked just fine. Both with original optical drive and with optibay. On the 17th, I just turned it on again in the morning (I think I had actually left it on the night before, while listening to a podcast while falling asleep), and it started again with the usual problem... and it doesn't solve if I try without optibay. Tried again my ssd on another macbook (externally) and it works fine. -Both times I sent it for checks it worked. Both times it had the original optical drive mounted in, it had been turned off for days (so I tried now to let it "rest" for 4 days, but nothing solved of course), and had took a ride on a plane..... either it likes flying, or I really don't know.... Any idea?