You need to go deeper than the upper intake manifold(UIM). Remove the lower intake manifold (LIM) too and change out the LIM gaskets for the new aluminum ones. The LIM gasket failure is just as common a problem as the perforated UIM as discussed in the techinfo article linked above by Archon.

This one:

http://www.bonnevilleclub.com/forum/...e=article&k=38

Pleas read this. It will explain the problems and lay out your options.

Did you only replace the UIM gasket, or the gasket and the entire UIM?