I have a mid 2012 MacBook Pro 13" non retina that remains on El Capitan. Two years to a year and a half ago, I installed a new battery, 16GB ram, and a 480GB SSD all from OWC. Since about March it has given me various issues. First the fans would run constantly and the idle % in Activity Monitor would go down to less than 1% when trying to Skype and Google or Photoshop and Google. Sometimes it would even go down to 0% idle and then freeze and restart. The whole computer would move very slowly before the freeze. I wouldn't get any messages when it restarted. This had lessened up a bit but then the additional issue came up that the idle percent would be just where it should be but everything would move extremely slowly. As in waiting minutes to even open Finder. This affected both going online and things like moving documents or selecting okay in a menu. I did a hardware test and it only gave me back a code related to the battery. I replaced the battery about a year and a half before any of this started. I put back in the original battery and did nothing else. The performance was the same. I did some research and I think (I may be wrong) that it is possibly the hard drive cable. I heard that was common on this model. I made an Apple appointment for this afternoon and to prepare I put back in the original 4GB ram and the original HDD (that is also set up with El Capitan). Now it works the same as it did way back before I replaced anything. It beachballs a little when doing some things but it is just how I remembered it performing with the HDD. The incredibly slow speeds are gone. I don't know if this supports or discredits the idea that it is the hard drive cable. I'm wondering if by putting back in the HDD I might have touched the cable just right so that it is now working? Any ideas about the cause of this and the possible cost to fix at Apple (or if I should do it myself)?