Is this a 2005? I had the same problem on my 2005 Mustang GT. Mine started out with the temp gauge sticking at cold. When the problem spread to the tach and it was off about 1500 rpm off, I had to get it fixed.
When you turn the car on the gauge needles swing back and forth to establish the initial position. If the stepper motor is sluggish, the gauge may find zero just about anywhere in the range. The gauge needle still goes up and down, but the zero point could be way off. Cold weather makes it worse.
I bought the stepper motors from a guy on ebay ($7.50 each) and replaced them myself. Not a difficult job if you are comfortable with soldering and desoldering and removing gauge needles, but quite tedious. The same guy will repair your cluster if you mail it to him, but you lose the cluster for a week or two.