This is just not good enough, as soon as the sun comes out your car goes wrong!!
Its got to go, I would not let them try another repair, you have given them enough chances. As Miopyk says get onto BMW directly and get them to get you a new one, that way they would have to give you a car in the meantime. If you leave it to Snows, they may not pass on your full anger at the situation, especially if a replacement comes from their normal allocation.