So we can rule out EGR. Sounds like a vacuum leak somewhere in the manifold vacuum system. Before throwing parts at it I would do a through search for vacuum leaks possibly related to the MAP Sensors. Did you look at both MAP Sensors for leaks or hoses off? Check out Haibecks article on the subject. Again sounds like the leak is in the manifold vacuum system not the secondary vacuum system.
http://www.zr1specialist.com/HAT%20W...5%20Engine.pdf
http://www.zr1specialist.com/HAT%20W...0the%20LT5.pdf
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