Alternator most likely. The 2G alternators on the fox bodies aren't even enough for the car stock and if it starts to degrade even just a little you'll get charging issues. Good practice for anyone with a fox is to upgrade to a 3G alternator out of a newer car. Will fix all your power issues and allow you to run underdrive pullies without problems.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.