I have a 92 F-150 with the 5.0L. I have been having an issue with the voltage dropping. At idle with no accessories on I have a normal reading on my voltage gauge inside the truck. I don't have a multimeter yet so i can't test the exact voltage. so, at idle my gauge sets on the "A" of the "normal" gauge. If I turn on my blower motor my voltage drops significantly. And the same with the lights. whenever i am driving and i come to a stop it also drops. when my voltage drops my rpm's drop with it. It has actually died on me twice. I got a new battery put in, i put a new voltage regulator and alternator brushes on it, (alternator tested good twice). I don't know what else to try.

 

Take the truck to the parts store and have them load test the battery and alternator on the truck.