I have a strange problem with the Mid 2012 (non retina) MBP 15 I recently purchased. When I bought it, the left shift control and alt keys were not registering key presses. All the other keys worked. I replaced the keyboard (that was fun) and nothing changed, to my surprise. I have done the usual SMC/PRAM resets to no avail. Short of using an external keyboard are there any other things to try? Could it be a dirty key board connector port, where the ribbon cable for the keyboard plugs into the mother board? Can that be cleaned? I am running Yosemite. Thanks in advance.