heat
 

I just bought a 99 mercury sable. I have 90,000 miles on the car. Problem is I am getting no heat and if I do it is luke warm. I changed the thermostat and burped the system. The antifreeze is rusty in color. The system over heats and will boil over, this only happens sometimes, when it does this the temperature red lines. It doesn't do this all the time. But after all I have done I still have no heat. Please help!!!! Winter in New England