Q: Truck over heats every winter

Every winter my truck seems to want to over heat on me. And every winter i put in a new thermostat and that seems to fix it for awhile. Last year i even did a coolant flush. Again this time of year comes around again and what happens? Over heated on me. But this time as it cooled down i could hear a faint hiss of fluid passing through somewhere and thats when the gauge dial would drop a little bit. What else could it be? I usually check the inlet and outlet of the heater core and both get warm

My car has 177577 miles.
My car has an automatic transmission.