Sags, and NO Support after 4 months

I purchased a Serta, iComfort Genius, King Mattress and foundation on November 20, 2012. After four months the foam is soft, saggy and offers no support. Mattress Firm sells this bed as a firm bed which it was for a few months. I filed and paid $25.00 for an inspection,a man came out and put a string across the middle of the bed and found a 1/2"sag. I told him it was a foam bed and I had been out of the bed for 7 hours. I asked him to push on the part of the bed I sleep on and feel how soft the foam had become. He said no. I only measure the sag. I explained that they were missing the point. You can feel the difference in the foam. It gives like an old hotel bed and offers no support. I then called Mattress Firm and explained that the foam recovers after you get out of it but the foam is no longer supportive and sags when you get on it (I weigh 200 lbs.). The service rep told me comfort is not a warranty issue. He told me to Call Serta and I did. Then Serta said they do not handle warranty issues unless there was no store involved, and I needed to contact Mattress Firm. DO NOT buy a Serta iComfort Genius because it loses its support in a very short time. DO NOT buy from Mattress Firm because they will not deal with the problem.