I bought an IMac two months ago and have already h...

I bought an IMac two months ago and have already had it fixed twice. Neither they nor Apple find the fault. They tell me that they cannot refund my money because more than 15 days have passed. After waiting 15 days, they return my computer saying that it is fine and that they have reviewed it. The computer is still bad. Why should I have seen it in those 15 days of testing if now not even they who are computer scientists see it. Very bad shopping experience. And this is not over yet.