FWIW,
The Vortec series of engines (96 to 99 - 4.3, 5.0, 5.7) have a plastic upper intake manifold but the lower portion is aluminium. The problem is with the intake manifold gasket (between the heads and IM), not the manifold(s) themselves. The OEM gasket was a defective design and they all have or will eventually fail.
Fel-Pro came out with a new IM gasket that cures the problem once and for all.
This is good advice. Expansion and contraction of a plastic intake can be a reason for vacuum leaks. Also could be the reason it starts ok cold then gives problems when it warms up. When running, try to spray some carb cleaner along the intake gaskets and see if the idle changes any. If it does, you have a bad gasket as mentioned above.
Also a firm believer in going back with AC Delco on the cap and rotor.
Clyde.
