Let’s try something, Disconnect the negative battery terminal, connect the jumper cables to the leads bypassing your battery and try to start, and then disconnect the negative jumper cable connection at the battery and see if it keeps running. This will test your charging system without the load of charging the battery; also could your battery be to small (not enough cold cranking amps)?
1985 F150 XLT Lariat 5.8L (351w) HO, "Holly built for Motorcraft" 4bbl, C6 Transmission, 4X2 extended cab, 8 foot bed.
"Some people spend a lifetime wondering if they've made a difference, Marines don't have that problem." President Ronald Regan 1985. I think he should have ended with: "Veterans don't have that problem." But that's me. D.A.V. USAF 1976-1992.