My mom has a 02 f150 with a 5.4. Everytime you get the rpm's over 3000 the lights dim and the battery light comes on. Then go back bright when you let off the gas. It has a new battery and a new Gatorback belt. Is the alternator going out? Or something else? Thanks

 

Alternator brushes are worn and not following at high rpm.
Replacement time.