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Charging system keeps cutting in and out

1979 chevy luv- charging system light keeps coming on-when it does this, the motor will run rough and not want to idle-system kicks in again, motor runs and idles great. new alternator and new regulator about 5000 miles ago, new battery and cables too. have had mechanics check system and they said output is as it should be-usually doesn’t misbehave for them or comes out of it right away. i’ve cleaned and tightend all connections-checked for broken wires(didn’t find any). any ideas why??

Japanese alternators like to quit at about 240,000 miles. Look for the engine to body ground wire. I bet it’s on the back of the engine. Go to the body end and scrape the paint off where it connects and reinstall it. After that, you’re on your own.

Could the drive belt be slipping?

I assume the belt has been checked/replaced and the belt is the correct application.
While a wiring diagram for this truck it not available to me, I’m kind of leaning towards something not related to the alternator and regulator.

Maybe the alternator light is a symptom caused by something like a corroded or burnt fusible link end, failing ignition switch, or the possibility of a bad fuse or fuse connection that provides power to the ignition or fuel pump.

I think these trucks use the old glass fuses and over time the solder in the end caps can melt a bit. With a bit of use the solder softens and moves around which can then lead to whatever is being powered to just up and quit. When the solder cools for a few seconds the connection is made good again and everything is fine until the next episode.

That’s probably what I would look at anyway and hope this helps.