what causes belt to turn unevenly? on 1993 Ford Crown Victoria

Car runs rough and makes a squealing noise. It does this about half of the time. The other half time it runs smoothly. Battery light is coming on.

Asked by for the 1993 Ford Crown Victoria
Sounds like the Alternator could be is failing and needs to be checked by a shop, You can find one here www.repairpal.com/directory But it could be anything driven by the drive belt Alternator, Water Pump, AC Compressor or Tensioner. Once they determine what needs replacement you can compare the cost here www.repairpal.com/estimator