1999 DODGE DURANGO

WHEN COLD OUT ( BELOW 40 DEG ) YOU CANT RUN THE HEAD LIGHTS AND HEATER AT SAME TIME. THE VOLTAGE DROPS AND THE CHECK GAGES LIGHT COMES ON. I HAVE REPLACED THE BATTERY AND HAD THE ALT CHECKED SEVREAL TIME IS ALWAYS TESTS OK. I EVEN SWAPPED ALT WITH MY OTHER TRUCK PROBLEM STAYED WITH THE DURRANGO, THE PROBLEM GOES AWAY WHEN THE TEMP GOES ABOVE 40 TO 50 DEG ???