Yesterday, my 89 lesabre overheated on the higway at -25 degree.It came back to normal,and was able to make it home.Afer refilling ti with antifreeze it ran ok and din''t overheat.Ithink that the coolant froze.But this morning when I took it for a ride,the temperature would stay low at idle and go to normal when running.So I chanced the thermostat,but it still does the same thing.Anybody can help?