I'm pretty sure that from late '95 to '04 GM put plastic intake manifold gaskets on the 3.4L V6, 4.3L V6, and the 5.7L V8. This supposed 5-year, no maintenance coolant contains a plasticizer. Over time, usually at about 100,000 miles (if driven hard) the intake manifold gaskets will start to deteriorate because of the extra chemicals in the Dex-Cool. Then after that, many bad things can happen. I've seen missfires, stuck valves, overheating, etc., its a nightmare.
Best thing to do is to flush it ALL out, a complete back flush, then install rubber Fel-Pro or equal intake manifold gaskets, and refill with glycol coolant (green)