2002 Ford Explorer

I have a 2002 Ford Explorer. The internal temperature system has been
fluctuating for the past few weeks. I normally set the internal
temperature at 72 degrees. The heating system will normally maintain this
temperature during the car trip. Recently, the system will change from 72
degrees to 71 or below. The temperature will always decrease. I will
reset the temp to 72 degrees and 20 to 30 to 60 seconds later, the
temperature will change again. This started happening in the 2nd half of
December. I live in Michigan.

What could be the problem and is it easy to fix or change?