60 gmc charging issues

[AngryPirate]

So, I've replaced the battery with an exide agm, had the generator rebuilt and replaced the starter.

If I charge the battery up with a charger, it will start.

If I boost it from another vehicle, it will start.

If I drive it for a while, it doesn't have enough juice to turn over the 305 v6.

Thoughts?

I want to say it's the external voltage regulator, but I'm not sure how to test that theory.

Thanks for any tips or ideas!