alternator no charge under boost

Started today going to church. 89 Mustang stock alternator. Got in boost battery light came on. Gauge showed no charge then back to normal.Tonight put vent on trans side of vac line to c-4 to get shift better, it is better. Done same thing every time I got into boost then went to normal when out of boost. One time did not come back to normal went back into boost came out then went to normal.Battery light and gauge agree with each other.Belt is tight on alternator and is still serpentine type. Haven't a clue what to check Needs someone smarter than me Help.Thank You

Found the problem was alternator they can go bad and not charge at higher rpm,s. Thought enveryone would want to know. thanks

I agree. I work in refrigeration industry. Wording is everything. Was noticing boost gauge not rpm when it happen. New alternator 6 months old or less didn't expect bad.Just a shadetree mechanic that is what I used to put motor in.Like you I have banged my head on problems. I'll dig deeper into problem more next time Thanks