My truck stopped charging. It had a new PCM 3 years ago and a new alternator last year. I didn't want to spend for a PCM so I went with the Larry B's regulator kit. It goes on the back of the alternator. So I hooked it up and it works great...for 5 minutes. When I start the truck cold in the morning it works, as it comes up to temp. It stops charging and the gen light comes on. When the truck cools down it works again. I'm pulling my hair out, what could this be?

 

Either a defective unit, bad solder, or a bad contact where you mounted it.I would suggest that you clean all the contacts your unit has to a metallic sheen, then apply a light coat of dielectric grease to them, and tighten all the bolts to spec.
And to check the wiring of the truck and your additions. ( Take care of the grounds, they are as important as the supply!)