The symptoms you're describing are classic for leaking intake gaskets. The original intake gasket is plastic with a silicone insert that does the actual sealing around the ports. Felpro makes an improved gasket that has a steel frame with a rubber insert and is far superior to the original gasket design. The Felpro part number is MS98002T. When replacing the gasket, it is extremely important to remove all traces of the old gasket, clean and oil the threads in the cylinder heads and the intake bolts, and use a torque wrench to tighten the bolts using the proper torque sequence.
If the cooling system has not had proper maintenance, the coolant ports of the intake manifold can corrode causing pitting. Slight pitting is generally not a problem and can be "leveled" using silicone sealer however, if the pits are severe enough, the manifold must be replaced.