On my '96 series 2, it occasionally uses coolant, and when I changed the Tstat, I noticed some coolant between the plastic intake manifold and the valve covers. Is it the intake that cracks, or does the intake manifold gasket that leaks?

Probably both. When my upper intake went bad it was actually warped not cracked. I talked to a guy at a local parts store and he said that GM put out a new improved upper intake plenum(or manifold). Either way you might have to change both.

What does the new and improved upper intake cost???