Are you using a PCV and breather system? this serves to relieve the pressure building up under the manifold. If so, check that your pcv valve is functioning properly.

I had the same problem, even with the breather/pcv. The rubber gaskets that seal the front and back of the manifold are junky and leak-prone. Best solution is to pull the manifold again, make sure the block and manifold are clean as can be, and instead of using the rubber gasket, just lay down a 1/4" bead of copper rtv sealant along the block, and a bit up the sides of the heads (right up to the manifold-to-head gasket). Drop the manifold back into place, and tighten to torque spec. This should give you a good seal. It worked for me.
Good luck...