Sounds like the voltage regulator which is not part of the alternator on the 5.0 up to 95. It is on the passenger side near the headlight, if it is a 2 wire alternator . I would check the voltage at the battery at idle, should be in the 14.6 range. if below than the regulator is crap if good, then there is a ground that is loose.
Alternator output amps for stock woukd be above 100 amps. The 630cca is the battery.
Think small and test first.