I have a problem that I can't quite figure out. My truck heater started blowing out cool-to luke-warm air at best. I first checked the antifreeze/coolant level first and it was a bit low. I topped off the radiator. I know the thermostat is operating properly. My temp gauge shows about 190 degrees when the engine is fully warmed up. The inlet heater gets hot, the outlet hose only barely warm. So I figure I have a clogged core. The wierd thing is, when I go up a steep hill, I get hot air coming out of the vent. It is as though when the heater core is at the lowest point in the system, the gravitational assist provided by that orientation allows for the core to fill with the hot fluid, providing the hot air flow. What is going on? Thanks!