The nylon LIM gaskets last about 80,000-120,000 miles on the 3800/3100/3400 engines. My experience with blown ones on the 3800, you will see coolant pool on the LIM. If you think that might be the issue (LIM gaskets), time for a pressure tester.

Just and FYI, Spring 2011, I tore the top of my Monte apart, modded it and threw in aluminum LIM gaskets. Sat for the winter, go to do some work on it spring of 2012, jack up the car, freakin Dex cool begins dripping!! I freaked, grabbed the pressure tester. Car NEVER lost pressure. Best I could figure, that coolant sat on a part of the car somewhere and never burned off, evaporated, nothing, just sat an entire summer of driving, winter sitting....