Is the idle to the point of almost dying then revving up and recovering? If not check the voltage at the back of the Alt. and see if the voltage drops there as well. If not I would suspect a bad connection in the cluster. I've seen both the voltmeter and oil pressure gauges on Rangers do this and were due to bad connection in the dash.
If the idle speed is dropping enough to effect the Alt. output then you'll need to figure out whats causeing this...Could be Ign, Fuel, Elect. or Vac problem.