CarMax Sold me a lemon. Irvine Nationwide

I am a college student and I work full time. I saved my money up to buy my first car and I chosed to go to Carmax. As a result my car has been in the shop almost every month since I had it. I bought it in 2013. The car shakes, cuts off, has had 4 spark plug issues, constant electrical problems and suspension problems. Everytime I take it in it's a two or three day process only to have the same thing happen over and over again. The warranty I paid $3k for is useless and on causes me more inconvenience. I almost lost my job and failed out of school because of attendance. All the while Carmax gives me the constant runaround and I make my payment ontime.