Starte/battery issues

[kevmic28]

Ok here it goes. 70 Gmc 350 with ac and elec fan, ran fine no issues other than heat soak on the starter when I parked it and tore it apart. Its back together a year later now but I am having issues.
My son drives it now, first truck.
Anyways, started having starting issues. I figured it was heat soak on the starter from the headers. So I replaced the starter. Truck ran fine for a month. Then started having starting issues. Had the 5 year old battery checked and it had a bad cell. New battery, back to starting fine.
He added a stereo and amp and back to starting issues. So we replaced the old alternator last weekend. Back to starting fine. Also replaced the negative batt line to the engine.
This past weekend it started having problems again. Replaced the starter again yesterday after having it tested and saying it was bad. Also added a heat shield. Started and ran fine yesterday and today until this evening.

Tonight he jumped in and it fired right up. He backed down the driveway, hit the brakes and it died. Would not start back up. Had to jump start it to get it back up the driveway.
Tried to start it again and nothing. Stereo off, ac off, elec fans off, had to sit for about 5 minutes jumping off from my Dodge ram for it to turn over.

Checked the pos lead to the starter and no power loss. Check the alternator line and no power loss from the alt to the bat.
Took the batt to get tested but they said it needed to be charged. Dead battery. So its on the chager now. Gonna get it tested again in the morning.

I am stumped. I cant find any drains. At 12.5 volts and it will start. 12.3 and no start. Also with engine running at idle the alt output sits at about 12.8 with the fan on and 13.8 with the fans off.

Bad starter or bad battery or what?