On your way to your lower intake manifold gaskets, check the date on your upper intake manifold, (UIM) as described in the techinfo article, to see if and when it might have been changed. Also, once you have removed the UIM, take a real close look around the EGR stovepipe passage, too. Take a needle, and poke it into the outside top of your UIM to see how hard the plastic should be. Now, take the needle and poke it into the EGR bore close to where it sits against the Lower Intake Manifold. Is it as hard as the outside, or is it soft? Does the needle go through? Is there any evidence of perforation? That area, and the coolant passage seals on the LIM gaskets are the two most likely places for internal coolant leaks to develop in your engine. If neither the LIM gaskets or the UIM have ever been changed, you should change both of them while you have it apart.