My 2002 Pathfinder will overflow coolant through the reservoir on the way from the city into the mountains. About an hour drive its mostly fine all the way till the last 10 miles which is also the biggest climb. I never have any problems in the city even when operating the A/C on 100 degree days on the freeway. It's only when I drive up 80 into the Tahoe area. I'll go weeks with no problems, no fluid loss. Then make a trip to Tahoe and it'll overflow. Also since its getting colder the heater is not as cold as it used to be and works better at high rpms. Any ideas?