Manifold/header/muffler/tail Pipe problems of the 1996 GMC Suburban

I have two GMC vehicles with the same engine, a gas 5. 7l v8, that required intake manifold gasket replacement within one month of each other. The first vehicle is a 1996 1500 Suburban with 90,000 miles. The other is a 1998 k1500 pickup with 80,000 miles. Both leaks occurred at the rear of the bottom intake manifold. The mechanic performing the repair states this is a very common problem with his shop performing approximately 5-6 a month in a town of only 30,000. The dealer denies any generic problem. Inspection of the gasket implies a temperature related problem of the gasket body that undermines the seal ridge.