Probably the regulator in the SBEC is going out. The regulator controls resistance to ground for the field winding.
RIP 91.5 W250 5 speed. The great experiment. 92 W250 HD 47RH with Compushift. Upgraded H1C to 62/60/16, Scheid Lightning VE, 60# valve springs, DAP 7X.010 injectors, 4" DE exhaust, home made cooler tubes. 01 2500 "the work truck".
Connections, connections, connections.
Always check the connections, hot (positive) side, as well as grounds.
Battery end, especially off the main terminal, and alternator end, ANYWHERE something can be loosened or broken by vibration.
You'd be surprised what can be torn loose, or loosened up, by an improperly secured battery.
Don't just bungee strap it down, either.
Get a real battery hold down.