When you remove the old gaskets, do you really clean the heads and the exhaust manifold? If you leave any parts of the old gasket on them, it will not permit a good seal. Do you start in the middle and tighten the manifold from center to the outer? Are the gaskets made for your specific set up and are they quality gaskets? If you keep having issues, have the manifolds looked at to make sure they are not warped. There also are torque tolerances. Have you tried using a torque wrench? All are suggestions, and I hope they help. Also check to make sure the exhaust pipes are secure and not swinging or moving the exhaust manifolds.