Never had this happen in the G8, but have seen it happen in other vehicles. The way we fixed it in the past was to remove the gas cap and let the pressure out (don't forget to put the cap back on). If you have a new bag of chips and drive from Denver to the mountains, the bag will look like a balloon. Same issue for pressurized gas tanks. For some reason, gaining a bunch of altitude causes problems with pressure in the tank. Do you still have the stock gas cap? You may need a newer one. Good luck and enjoy Colorado!
2009 Panther Black, Premium Package, Breyton 19" wheels, General GMax AS03 tires.
1994 Ford Bronco
2002 Honda VTX 1800