If it randomly started doing this I would make sure you did not run over a bag or something that is restricting the air flow to the radiator (sometimes they can make their way in places you would never think of). Next I would check the thermostat because you know your fans are working. If the thermostat is working properly then the only thing left would be your water pump. I don't know much about the G6 water pumps when they are worn out will leak a little out a small hole on the bottom of where the gear is internally. Hope this helps.
2000 Trans Am WS6: Sebring Silver Metallic LS6, TEA 5.3 stg. 2 heads, VRx5 cam, Monster stg 3 clutch, 3.73 gears, yada yada.....too much more to list.
Daily Driver- 2003 Silverado Z71, 3 inch body lift, 35s, Hypertech
Wife's: 2010 G6 Silver Metallic 3.5 V6 with ebony int.