My mid-2012 MBP has been insanely slow pretty much since I bought it. Constant pinwheels of death. I've had it looked at, at the apple store 3 times. Upgraded to yosemite... repaired disk utility, and cleaned off uneeded programs. Still constant pinwheels, and insanely slow. Only 4GB... so... will more RAM fix this machine? And.. if so... how much RAM can I upgrade to? And, where to buy it, and how do I install it? Thanks!