Grand Prix series 3 engine

I am wondering what they upgrade on series 3 engine.
I read that series 3 has new intake manifold so it should solve problems from series 2.
However my series 3 intake manifold needed service at 80k miles.

The manifold itself is aluminum cast, a big upgrade from the plastic plenum. The plastic intake would warp and crack causing issues. What I think you are referring to is the intake gasket. You probably had the old style plastic framed gasket on there. Those were a horrible design and would crack into a million pieces. The new aluminum framed gaskets are extremely durable. Very rarely once you upgrade to them will you have to do it again.